feat(ef): the Mass preface, RG 482-499

Item 2 of Phase 3 (celebrant-rubrics-phase1), the bulk of this phase
and the last EF Mass rubric this project scoped: which preface is
said, deferring to seasonal/proper-title rules for the fourteen named
prefaces, then RG 498's Common residual.

RG 482 gives the resolution chain: the Mass's own proper preface,
failing that the seasonal one, failing that Common. Read literally,
RG 484-497 look like fourteen separate rules, but each numbered
rubric's own propria/de-Tempore pair produces the SAME preface
identity either way, so the whole chain collapses into one priority-
ordered decision: title/mystery triggers (Holy Cross, Sacred Heart,
Christ the King, Trinity, St Joseph, BVM, the Nativity octave, the
Apostles, Epiphany), each independent of season, then six seasonal
windows (Nativity, Epiphany, Lent, Holy Cross/Passiontide, Easter,
Ascension, Holy Spirit, Trinity-for-ordinary-Sundays), then Common.

The "one genuinely unproven piece" the design spec worried about --
per-feast proper prefaces extracted from the Missal's propers at scale
-- turned out not to be a large-scale extraction problem at all: every
one of the fourteen propers is a closed, small, subject/slug-keyed
trigger (mirroring Precedence_ef.band's own RG 91 table), not
thousands of individual saints' pages. The one genuine open question
(RG 488, the Chrism Mass) is N/A: this engine resolves one Mass per
civil day and has no separate Chrism-Mass dimension.

Preface is a new kernel type (lib/kernel/preface.ml[i]), not an
EF-specific one, the same placement as Colour/Subject/Mass_formulary:
Liturgical_day.t is parameterised only over season/rank, so any field
it carries generically must live in the kernel even though only EF
constructs a value of it today. Wired exactly as creed/gloria were
(Rite.t.preface, Calendar.ml, Rite_ef.context), but Preface.t option,
not a bare bool: unlike creed/gloria, a preface is said only at a
Mass, and Good Friday (1955-restored Holy Week) resolves an observed
celebration but has no Mass at all -- None is the honest answer there,
and also the neutral value an unimplemented rite returns.

Two priority-order findings only the oracle settled, not derivable
from the Latin text in isolation, both cross-checked against 358
individually classified entries in the FIUV Ordo's own praef column
(test/fixtures/fiuv-ordo-2025-2026.sexp, already captured, wired up
here as a new comparison axis): RG 484(b)'s own "except Masses with a
proper of the divine mysteries or Persons" is narrower than every
other window's implicit exception (an Apostle inside the Nativity
octave is overridden to Nativity; outside it, keeps his own preface
even inside another window); and RG 495's "et votivis" half is live
for the one office this engine models without a votive-Mass dimension
(the Saturday Office of the BVM), which also proved a vigil is not a
"festum" for this purpose (the Assumption's own vigil takes Common,
not BVM, correcting an initial reuse of Precedence_ef.marian_slugs
that had no reason to make that distinction for its own, different
rubric). A third, RG16(a)-shaped fix landed the same way: RG 494(b)'s
own Trinity grant must read the day's TEMPORAL season, not the
celebration that actually won it, or a Class1 feast with no preference
of its own (All Saints) wrongly falls to Common on a Sunday it merely
commemorates.

colitur rubrics gains a sixth TAB-separated column. Domain-wide
1583-9999 exhaustive sweep confirms every Christmastide day resolves
Nativity/Epiphany/Bvm, every Paschaltide day one of its own three
windows or a season-independent title, every Lent day Lent or a
title, and Passiontide legitimately produces Easter exactly once a
year (the Vigil Mass) -- 8416 of the domain's 8417 years, the one
short year being the domain's own upper boundary (the liturgical year
opening in 9999 cannot construct dates in year 10000, a pre-existing
edge this sweep re-confirms rather than a new one).

day/readings verified byte-identical to the branch's own state before
this phase (commit 9c96e0a) across a 455-year sample spanning the
whole domain -- not literally to the v0.10.1 tag, which 22 earlier
commits on this same branch (Phase 1/2, the bissextile shift, the
Rogation colour fix) had already moved past before this phase began.

11 files changed, 676 insertions, 1 deletions

diff --git a/lib/kernel/calendar.ml b/lib/kernel/calendar.ml
index 7178780..a1309f9 100644
--- a/lib/kernel/calendar.ml
+++ b/lib/kernel/calendar.ml
@@ -554,6 +554,9 @@ let build_day (rite : ('s, 'r) Rite.t) (idx : 'r Layer.index)
   let gloria =
     rite.Rite.gloria ~temporal ~observed:resolution.Precedence.observed.Precedence.cel ~date
   in
+  let preface =
+    rite.Rite.preface ~temporal ~observed:resolution.Precedence.observed.Precedence.cel ~date
+  in
   {
     Liturgical_day.date;
     rite = rite.Rite.id;
@@ -568,6 +571,7 @@ let build_day (rite : ('s, 'r) Rite.t) (idx : 'r Layer.index)
     formulary;
     creed;
     gloria;
+    preface;
   }
 
 let year (rite : ('s, 'r) Rite.t) (layer : 'r Layer.t) (y : int) :
diff --git a/lib/kernel/liturgical_day.ml b/lib/kernel/liturgical_day.ml
index 98c5084..c3ecb37 100644
--- a/lib/kernel/liturgical_day.ml
+++ b/lib/kernel/liturgical_day.ml
@@ -28,5 +28,6 @@ type ('s, 'r) t = {
           a rite with no lectionary -- see {!Mass_formulary} *)
   creed : bool;  (** whether the Creed is said at this day's Mass; see {!Rite.t.creed} *)
   gloria : bool;  (** whether the Gloria in excelsis is said; see {!Rite.t.gloria} *)
+  preface : Preface.t option;  (** which preface is said; see {!Rite.t.preface} *)
 }
 [@@deriving sexp]
diff --git a/lib/kernel/liturgical_day.mli b/lib/kernel/liturgical_day.mli
index be17d73..643097b 100644
--- a/lib/kernel/liturgical_day.mli
+++ b/lib/kernel/liturgical_day.mli
@@ -51,5 +51,12 @@ type ('s, 'r) t = {
       (** Whether the Gloria in excelsis is said at this day's Mass --
           {!Rite.t.gloria}, EF: RG 431-432. Same seam as [creed] in every
           respect. *)
+  preface : Preface.t option;
+      (** Which preface is said at this day's Mass -- {!Rite.t.preface},
+          EF: RG 482-499. Same seam as [creed]/[gloria], but [option], not
+          a bare {!Preface.t}: see {!Rite.t.preface}'s own citation for why
+          -- [None] both for a rite that has not implemented the rule and
+          for a real day this engine resolves that has no Mass at all
+          (Good Friday). *)
 }
 [@@deriving sexp]
diff --git a/lib/kernel/preface.ml b/lib/kernel/preface.ml
new file mode 100644
index 0000000..08d00b4
--- /dev/null
+++ b/lib/kernel/preface.ml
@@ -0,0 +1,59 @@
+(* RG 482-499 -- see the .mli for the rubric quoted in full and the
+   placement rationale. Argument-less variants only, so no
+   [Sexplib0.Sexp_conv] open is needed -- see CLAUDE.md's own gotcha note. *)
+type t =
+  | Nativity
+  | Epiphany
+  | Lent
+  | Holy_cross
+  | Easter
+  | Ascension
+  | Sacred_heart
+  | Christ_the_king
+  | Holy_spirit
+  | Trinity
+  | Bvm
+  | St_joseph
+  | Apostles
+  | Common
+  | Requiem
+[@@deriving sexp]
+
+let all =
+  [ Nativity; Epiphany; Lent; Holy_cross; Easter; Ascension; Sacred_heart; Christ_the_king;
+    Holy_spirit; Trinity; Bvm; St_joseph; Apostles; Common; Requiem ]
+
+let to_string = function
+  | Nativity -> "nativity"
+  | Epiphany -> "epiphany"
+  | Lent -> "lent"
+  | Holy_cross -> "holy-cross"
+  | Easter -> "easter"
+  | Ascension -> "ascension"
+  | Sacred_heart -> "sacred-heart"
+  | Christ_the_king -> "christ-the-king"
+  | Holy_spirit -> "holy-spirit"
+  | Trinity -> "trinity"
+  | Bvm -> "bvm"
+  | St_joseph -> "st-joseph"
+  | Apostles -> "apostles"
+  | Common -> "common"
+  | Requiem -> "requiem"
+
+let of_string = function
+  | "nativity" -> Some Nativity
+  | "epiphany" -> Some Epiphany
+  | "lent" -> Some Lent
+  | "holy-cross" -> Some Holy_cross
+  | "easter" -> Some Easter
+  | "ascension" -> Some Ascension
+  | "sacred-heart" -> Some Sacred_heart
+  | "christ-the-king" -> Some Christ_the_king
+  | "holy-spirit" -> Some Holy_spirit
+  | "trinity" -> Some Trinity
+  | "bvm" -> Some Bvm
+  | "st-joseph" -> Some St_joseph
+  | "apostles" -> Some Apostles
+  | "common" -> Some Common
+  | "requiem" -> Some Requiem
+  | _ -> None
diff --git a/lib/kernel/preface.mli b/lib/kernel/preface.mli
new file mode 100644
index 0000000..b43890e
--- /dev/null
+++ b/lib/kernel/preface.mli
@@ -0,0 +1,50 @@
+(** The Mass preface (Missale Romanum, Rubricae Generales, Caput XVII("De
+    Ritibus servandis in celebratione Missae"), "H) De praefatione", RG
+    482-499; docs/research/LT.txt, grep "praefatione dicitur quae cuique").
+
+    RG 482 gives the resolution chain in full: {i "Praefatio dicitur quae
+    cuique Missae propria est; qua deficiente, dicitur praefatio de
+    Tempore, secus communis"} -- the Mass's own PROPER preface; failing
+    that, the SEASONAL ("de Tempore") one; failing that, the COMMON. RG
+    483: {i "Nulla commemoratio, in Missa occurrens, praefationem propriam
+    inducit"} -- a commemoration never induces its own proper preface,
+    which is why {!Rite_ef.Rubrics_ef.preface} (the one rite that
+    implements this so far) reads only the day's OBSERVED celebration,
+    never its commemorations, the identical discipline {!Rite.t.creed}'s
+    own RG 476(e) already established.
+
+    RG 484-497 enumerate the fourteen named propers -- Nativity, Epiphany,
+    Lent, Holy Cross, [the Chrism Mass, out of scope: see the .ml's own
+    N/A note], Easter, Ascension, the Sacred Heart, Christ the King, the
+    Holy Spirit, the Trinity, the Blessed Virgin Mary, St Joseph, the
+    Apostles. RG 498 is the residual Common; RG 499 the Requiem.
+
+    A KERNEL type, not a rite-specific one (the same placement as
+    {!Colour}, {!Subject}, {!Mass_formulary}): {!Liturgical_day.t} is
+    parameterised only over a rite's season/rank, so any field it carries
+    generically must live here, even though only the EF rite constructs a
+    value of it today -- CLAUDE.md's own "generalise when forced, not
+    speculatively" stance, already the precedent {!Mass_formulary}
+    follows. *)
+
+type t =
+  | Nativity  (** RG 484 *)
+  | Epiphany  (** RG 485 *)
+  | Lent  (** RG 486, "de Quadragesima" *)
+  | Holy_cross  (** RG 487, "de sancta Cruce" *)
+  | Easter  (** RG 489, "paschalis" *)
+  | Ascension  (** RG 490 *)
+  | Sacred_heart  (** RG 491, "de Ss.mo Corde Iesu" *)
+  | Christ_the_king  (** RG 492, "de D. N. Iesu Christo Rege" *)
+  | Holy_spirit  (** RG 493, "de Spiritu Sancto" *)
+  | Trinity  (** RG 494, "de Ss.ma Trinitate" *)
+  | Bvm  (** RG 495, "de beata Maria Virgine" *)
+  | St_joseph  (** RG 496, "de S. Ioseph" *)
+  | Apostles  (** RG 497, "de Apostolis" *)
+  | Common  (** RG 498, "communis" -- the residual when nothing else applies *)
+  | Requiem  (** RG 499, "defunctorum" *)
+[@@deriving sexp]
+
+val all : t list
+val to_string : t -> string
+val of_string : string -> t option
diff --git a/lib/kernel/rite.ml b/lib/kernel/rite.ml
index 915941d..e44c2c6 100644
--- a/lib/kernel/rite.ml
+++ b/lib/kernel/rite.ml
@@ -21,4 +21,5 @@ type ('s, 'r) t = {
     Mass_formulary.t option * Citation.t list;
   creed : temporal:('s, 'r) Temporal.t -> observed:'r Celebration.t -> date:Date.t -> bool;
   gloria : temporal:('s, 'r) Temporal.t -> observed:'r Celebration.t -> date:Date.t -> bool;
+  preface : temporal:('s, 'r) Temporal.t -> observed:'r Celebration.t -> date:Date.t -> Preface.t option;
 }
diff --git a/lib/kernel/rite.mli b/lib/kernel/rite.mli
index b9d1961..d53e4e6 100644
--- a/lib/kernel/rite.mli
+++ b/lib/kernel/rite.mli
@@ -140,4 +140,21 @@ type ('s, 'r) t = {
           parameters and the same reasons for each, a [bool] not an
           [option], and [false] is the answer a rite that has not
           implemented the rule returns explicitly. *)
+  preface : temporal:('s, 'r) Temporal.t -> observed:'r Celebration.t -> date:Date.t -> Preface.t option;
+      (** Which preface is said at this day's Mass (EF: RG 482-499). Same
+          seam and same three parameters as {!creed}/{!gloria}, for the
+          same reasons -- {!Preface}'s own citation has the rubric in full.
+          [Preface.t option], not a bare [Preface.t]: unlike [creed]/
+          [gloria], where "not said" is itself a legitimate answer a
+          [bool] can carry, a preface is said only at a MASS, and this
+          engine constructs at least one day (Good Friday, the
+          1955-restored Holy Week) that resolves an [observed] celebration
+          but has no Mass at all -- [None] is the honest answer there, not
+          a manufactured preface. [None] is also the value a rite that has
+          not implemented this rule returns, the same "the type's own
+          neutral value" contract {!creed}/{!gloria} give for [false] --
+          the two meanings ("this rite does not model the question" and
+          "this specific day has no Mass to preface") collapse onto the
+          same representation deliberately: a caller with no rite-specific
+          context to distinguish them should not need one. *)
 }
diff --git a/lib/rites/rite_ef/rite_ef.ml b/lib/rites/rite_ef/rite_ef.ml
index d7b3d83..808e017 100644
--- a/lib/rites/rite_ef/rite_ef.ml
+++ b/lib/rites/rite_ef/rite_ef.ml
@@ -52,4 +52,5 @@ let context ~lectionary ~commons : (Vocab_ef.season, Vocab_ef.rank) Rite.t =
     transfer_target = Precedence_ef.transfer_target;
     readings = Lectionary_ef.readings ~lectionary ~commons;
     creed = Rubrics_ef.creed;
-    gloria = Rubrics_ef.gloria }
+    gloria = Rubrics_ef.gloria;
+    preface = Rubrics_ef.preface }
diff --git a/lib/rites/rite_ef/rite_ef.mli b/lib/rites/rite_ef/rite_ef.mli
index 7025143..2ed61de 100644
--- a/lib/rites/rite_ef/rite_ef.mli
+++ b/lib/rites/rite_ef/rite_ef.mli
@@ -39,6 +39,10 @@ module Rubrics_ef = Rubrics_ef
     - [gloria]: {!Rubrics_ef.gloria}, RG 431-432 -- whether the Gloria in
       excelsis is said. Reads {!Rubrics_ef.te_deum} (Breviary nn. 237-238)
       for RG 431(a)/432(a)'s own deferral.
+    - [preface]: {!Rubrics_ef.preface}, RG 482-499 -- which preface is
+      said. Same seam as [creed]/[gloria]; [None] both for a day with no
+      Mass at all (Good Friday) and for a rite that has not implemented
+      the rule.
 
     Deliberately carries no [sanctoral]/[lectionary] fields the way the
     original design-doc sketch of [RITE] does: {!Colitur_kernel.Rite.t} (the
diff --git a/lib/rites/rite_ef/rubrics_ef.ml b/lib/rites/rite_ef/rubrics_ef.ml
index a497b57..eea92a9 100644
--- a/lib/rites/rite_ef/rubrics_ef.ml
+++ b/lib/rites/rite_ef/rubrics_ef.ml
@@ -697,3 +697,499 @@ let gloria ~(temporal : (Vocab_ef.season, Vocab_ef.rank) Temporal.t)
        Gloria mirrors [te_deum] exactly for every Mass not already decided
        above. This is the ONE call site [te_deum] exists to serve. *)
     te_deum ~temporal ~observed ~date
+
+(* Missale Romanum, Rubricae Generales, Caput XVII, "H) De praefatione" (RG
+   482-499; docs/research/LT.txt, grep "praefatione dicitur quae cuique"),
+   quoted here in full so every branch below can cite its own paragraph
+   without re-quoting the whole rubric:
+
+     "482. Praefatio dicitur quae cuique Missae propria est; qua deficiente,
+   dicitur praefatio de Tempore, secus communis.
+
+     483. Nulla commemoratio, in Missa occurrens, praefationem propriam
+   inducit.
+
+     484. Praefatio de Nativitate Domini dicitur:
+       a) tamquam propria in Missis de Nativitate Domini et de eiusdem
+   octava, necnon in festo Purificationis B. Mariae Virg.;
+       b) tamquam de Tempore, infra octavam Nativitatis Domini, etiam in
+   Missis quae secus praefationem propriam haberent, exceptis iis Missis
+   quae praefationem propriam de divinis mysteriis vel Personis habent; et
+   a die 2 ad 5 ianuarii.
+
+     485. Praefatio de Epiphania Domini dicitur:
+       a) tamquam propria in Missis de festo Epiphaniae et de
+   Commemoratione Baptismatis D. N. Iesu Christi;
+       b) tamquam de Tempore diebus a 7 ad 13 ianuarii.
+
+     486. Praefatio de Quadragesima dicitur:
+       a) tamquam propria in Missis de Tempore a feria IV cinerum usque ad
+   sabbatum ante dominicam I Passionis;
+       b) tamquam de Tempore in ceteris Missis quae celebrantur eodem
+   tempore, et praefatione propria carent.
+
+     487. Praefatio de sancta Cruce dicitur:
+       a) tamquam propria in Missis de tempore a dominica I Passionis usque
+   ad feriam V in Cena Domini; in Missis tam festivis quam votivis de
+   sancta Cruce, de Passione Domini et instrumentis Passionis Domini, de
+   pretiosissimo Sanguine D. N. Iesu Christi, de Ss.mo Redemptore;
+       b) tamquam de Tempore in omnibus Missis a dominica I Passionis usque
+   ad feriam IV Hebdomadae sanctae, quae praefatione propria carent.
+
+     488. Praefatio de Missa chrismatis dicitur feria V in Cena Domini, in
+   sua Missa.
+
+     489. Praefatio paschalis dicitur:
+       a) tamquam propria in Missis de Tempore a Missa Vigiliae paschalis
+   usque ad vigiliam Ascensionis Domini;
+       b) tamquam de Tempore in ceteris Missis quae celebrantur eodem
+   tempore, et praefatione propria carent.
+
+     490. Praefatio de Ascensione Domini dicitur:
+       a) tamquam propria in festo Ascensionis Domini;
+       b) tamquam de Tempore in omnibus Missis a feria VI post Ascensionem
+   usque ad feriam VI ante vigiliam Pentecostes, quae praefatione propria
+   carent.
+
+     491. Praefatio de Ss.mo Corde Iesu dicitur in Missis festivis et
+   votivis de Ss.mo Corde Iesu.
+
+     492. Praefatio de D. N. Iesu Christo Rege dicitur in Missis festivis
+   et votivis de D. N. Iesu Christo Rege.
+
+     493. Praefatio de Spiritu Sancto dicitur:
+       a) tamquam propria in Missis de Tempore a vigilia Pentecostes usque
+   ad subsequens sabbatum; et in Missis festivis et votivis de Spiritu
+   Sancto;
+       b) tamquam de Tempore in ceteris Missis quae celebrantur eodem
+   tempore, et praefatione propria carent.
+
+     494. Praefatio de Ss.ma Trinitate dicitur:
+       a) tamquam propria in Missis de festo et votivis Ss.mae Trinitatis;
+       b) tamquam de Tempore in dominicis Adventus, et in omnibus dominicis
+   II classis, extra tempus natalicium et paschale.
+
+     495. Praefatio de beata Maria Virgine dicitur in Missis festivis et
+   votivis beatae Mariae Virginis, praeterquam in festo Purificationis B.
+   Mariae Virg.
+
+     496. Praefatio de S. Ioseph dicitur in Missis festivis et votivis S.
+   Ioseph.
+
+     497. Praefatio de Apostolis dicitur in Missis festivis et votivis
+   Apostolorum et Evangelistarum.
+
+     498. Praefatio communis dicitur in Missis quae praefatione propria
+   carent, nec sumere debent praefationem de Tempore.
+
+     499. Praefatio defunctorum dicitur in Missis defunctorum."
+
+   THE SHAPE OF THE RULE, once, rather than at every branch: RG 482's own
+   chain is "propria, else de Tempore, else communis". Read literally, 484-
+   497 look like FOURTEEN SEPARATE RULES, but on inspection each numbered
+   rubric's own (a)/(b) pair (where it has both) produces the SAME preface
+   identity either way -- (a) is the propria reading ("this Mass's OWN
+   preface"), (b) is the de-Tempore reading ("this OTHER Mass, lacking one
+   of its own, borrows it") -- so for the single question this function
+   answers (WHICH preface, not WHETHER it counts as propria or de Tempore
+   for some other purpose) the two halves collapse into one PRIORITY-
+   ORDERED decision: a fixed list of "genuinely proper" triggers (title/
+   mystery feasts, independent of season), checked first in a citable
+   order, falling through to a fixed list of SEASONAL windows, falling
+   through to [Common]. RG 483 (a commemoration never induces a proper)
+   holds by construction, the same way [creed]'s own 476(e) does: every
+   branch below reads only [observed], never a day's admitted
+   commemorations.
+
+   THE PRIORITY ORDER ITSELF was cross-checked against 358 real,
+   individually classifiable entries in the FIUV Ordo's own [praef] column
+   (test/fixtures/fiuv-ordo-2025-2026.sexp, test_fiuv_ordo.ml) spanning the
+   WHOLE liturgical year -- not merely derived from the Latin text in
+   isolation. Two findings the plain text alone would not have settled,
+   both empirically confirmed rather than assumed:
+
+     - 484(b)'s own "exceptis iis Missis quae praefationem propriam de
+       divinis mysteriis vel Personis habent" is NARROWER than every other
+       window's implicit "unless it already has a genuine proper" -- St
+       John the Evangelist (27 December, on {!creed_apostle_slugs}, so his
+       OWN Apostles preface (497) would otherwise apply) is overridden to
+       [Nativity] inside the octave (confirmed: the Ordo's own 27 December
+       entry reads "de Nativ.", not "App."), while St Barnabas/Sts Philip &
+       James/the other Apostles OUTSIDE the octave keep their own Apostles
+       preface even inside another window (Sts Philip & James, 11 May,
+       inside the Easter window: confirmed "App." in the Ordo, not
+       "Pasch."). So [Apostles] is checked AFTER the Nativity window below,
+       but every OTHER title trigger (Holy Cross/Sacred Heart/Christ the
+       King/Trinity/St Joseph/BVM) is checked BEFORE it -- RG 484(b)'s own
+       narrower carve-out, read literally: unreachable on the shipped
+       calendar for the other five (no such feast falls 25 December-5
+       January), so this ordering is defensive for them, not observed live,
+       the same "checked, not merely assumed" discipline
+       {!Precedence_ef.marian_slugs}'s own citation follows elsewhere.
+     - RG 495's own "et votivis" half is live on this engine's data after
+       all, for the ONE office this project already models as a votive-
+       shaped Mass without a votive-Mass DIMENSION (RG 78/91 entry 27, the
+       Saturday Office of the BVM, {!Temporal_ef}'s own [subject = Bvm]
+       tag): confirmed directly (3 January and 10 January 2026, both the
+       Saturday Office, both read "BMV" in the Ordo) -- including on 3
+       January, itself inside the Nativity's own "2 ad 5 ianuarii" de-
+       Tempore window, where BVM still wins, corroborating the same
+       "genuine propria outranks every window" ordering the Apostles
+       finding above established from the opposite direction.
+
+   Good Friday's own printed [praef] text ("comm. Feria VI prima in
+   mense.") was NOT used to check this function's own [None] answer for
+   that day: {!test_fiuv_ordo.ml}'s own F1 (Gloria) already adjudicated
+   this exact date's raw text as unreliable (a copied, not a considered,
+   line -- see that allow-list entry's own citation for the full argument,
+   confirmed against the PDF's own page image, not merely the extracted
+   fixture) -- the same defect, read again, would apply equally to
+   whatever trails "praef." on the identical corrupted line, so this
+   function's [None] rests on RG 28's own "no Mass" structural argument
+   alone (the same argument [creed]/[gloria]/[te_deum] already give for
+   this date), not on any Ordo corroboration. *)
+
+(* RG 487(a): the two GENUINE fixed-date feast triggers in the shipped
+   universal calendar -- the Exaltation of the Holy Cross (14 September)
+   and the Most Precious Blood (1 July), both [subject = Lord]. RG 487(a)'s
+   own further-named categories ("de Passione Domini et instrumentis
+   Passionis Domini, de Ss.mo Redemptore") have NO corresponding entry
+   anywhere in data/ef/sanctoral.sexp (checked directly, grepping for
+   "instrument"/"redeem": zero hits) -- genuinely absent from the shipped
+   1962 universal calendar, not merely unmatched by this list, so they are
+   N/A rather than silently unreachable. *)
+let preface_holy_cross_slugs =
+  [ "exaltation-of-the-holy-cross"; "precious-blood-of-our-lord-jesus-christ" ]
+
+(* RG 496: the two St Joseph feasts in the shipped calendar (19 March, 1
+   May) -- both [subject = Saint], so (unlike RG 495's own BVM feasts) no
+   [subject]-based fallback exists or is needed; this closed list is the
+   whole of what RG 496 can ever reach on shipped data. *)
+let preface_st_joseph_slugs = [ "joseph-spouse-of-the-bl-virgin-mary"; "joseph-the-workman" ]
+
+(* RG 497's own [Apostolorum et Evangelistarum] population is WIDER than
+   {!creed_apostle_slugs}: RG 475(e) is restricted to a NATALICIUM ("festis
+   NATALICIIS Apostolorum...", that module's own citation), but RG 497 has
+   no such restriction at all ("in Missis festivis et votivis Apostolorum
+   et Evangelistarum" -- ANY festive/votive Mass of an Apostle or
+   Evangelist). FOUND, not assumed: the FIUV Ordo's own 30 June entry
+   ("In Commemoratione S. Pauli Ap.", data/ef/adjustments.sexp's own RG
+   110(c) [Add], {!Precedence_ef}'s own citation -- a genuine [Feast]-
+   status office of Paul the Apostle, but NOT his own dies natalis, so
+   {!creed_apostle_slugs} deliberately excludes it) reads "App. I", not
+   "comm." -- checked directly while building this comparison, not
+   guessed. [creed_apostle_slugs] itself is UNCHANGED (RG 475(e)'s own
+   narrower "natalicium" reading still holds for the Creed); this is a
+   SEPARATE, wider list for RG 497 alone.
+
+   "conversion-of-st-paul" (25 January, Class3, the SAME "not a
+   natalicium" shape {!creed_apostle_slugs}'s own citation excludes it
+   for) is a plausible SECOND candidate by the identical RG 497 reasoning
+   -- deliberately NOT added: it is UNWITNESSED (25 January falls on a
+   Sunday, hence impeded, in the one fixture year this engine's Ordo
+   evidence covers, {!test_fiuv_ordo.ml}'s own window), and "a wrong
+   citation is worse than a missing one" ({!Precedence_ef.marian_slugs}'s
+   own citation, the precedent this follows). Left for a future fixture
+   year to confirm or refute. *)
+let preface_apostle_slugs = "in-commemoratione-sancti-pauli-apostoli" :: creed_apostle_slugs
+
+(* RG 495's own [beatae Mariae Virginis] population is also WIDER than
+   {!Precedence_ef.marian_slugs}: that list was built for a DIFFERENT
+   rubric (RG 112(d), whether a commemoration invokes HER OWN
+   intercession specifically) with a correspondingly narrower, oration-
+   checked standard, and its own citation explicitly EXCLUDES "dedication-
+   of-the-basilica-of-st-mary-major" (5 August) for exactly that reason --
+   "whose own oration could not be found... to confirm it invokes her
+   intercession". RG 495 asks a different, WIDER question ("is this Mass
+   festive or votive OF the Blessed Virgin Mary at all"), which the
+   Dedication of St Mary Major answers on its own title alone, without
+   needing the oration-level standard RG 112(d) requires. FOUND, not
+   assumed: the FIUV Ordo's own 5 August entry reads "BMV Et te in
+   Festivitate.", not "comm." -- checked directly. *)
+let preface_bvm_slugs = "dedication-of-the-basilica-of-st-mary-major" :: Precedence_ef.marian_slugs
+
+let preface ~(temporal : (Vocab_ef.season, Vocab_ef.rank) Temporal.t)
+    ~(observed : Vocab_ef.rank Celebration.t) ~(date : Date.t) : Preface.t option =
+  let easter = Computus.gregorian_easter (Date.year date) in
+  let n = Date.to_rata date - Date.to_rata easter in
+  let m = Date.month date and dd = Date.day date in
+  let slug = Slug.to_string observed.Celebration.slug in
+  if
+    (* RG 28-34/RG 23(b), the same structural "no Mass at all" position
+       [creed]'s own vigil comment and [gloria]'s own 432(d) comment both
+       take for Good Friday specifically: the 1955-restored Holy Week has
+       no Mass whatsoever that day (only the afternoon liturgical action),
+       so there is no Mass to preface. Checked ahead of the
+       {!Colour.Black} Requiem proxy immediately below -- unlike All
+       Souls, Good Friday sharing that colour is coincidental, not
+       diagnostic (temporal_ef.ml's own RG 132 citation), and the two need
+       DIFFERENT answers here (unlike [creed]/[gloria]/[te_deum], where
+       both collapse to the same boolean) -- so this function cannot reuse
+       their shared single guard and must split Good Friday out first. *)
+    n = -2
+  then None
+  else if
+    (* RG 499: "in Missis defunctorum" -- the same {!Colour.Black} proxy
+       [creed]'s own 476(f), [te_deum]'s own 238(d) and [gloria]'s own
+       432(d) already use (this file's own header has the full argument
+       and the two-member population this proxy rests on). With Good
+       Friday split out above, the one remaining member is All Souls. *)
+    observed.Celebration.colour = Colour.Black
+  then Some Preface.Requiem
+  else if
+    (* RG 487(a)'s own fixed-feast half -- checked first among the title
+       triggers per this file's own header (arbitrary among these six,
+       since none can ever co-occur with another on shipped data; Holy
+       Cross is placed first only because it is also the anchor for the
+       Passiontide WINDOW checked later below, keeping both citations
+       adjacent in this file). *)
+    List.mem slug preface_holy_cross_slugs
+  then Some Preface.Holy_cross
+  else if
+    (* RG 491: "in Missis festivis... de Ss.mo Corde Iesu" -- the Friday
+       after the Octave of Corpus Christi (Easter+68), {!Temporal_ef}'s own
+       named slug. Confirmed against the Ordo (12 June 2026: "de Ss.mi
+       Corde Iesu"). *)
+    slug = "ef-sacred-heart"
+  then Some Preface.Sacred_heart
+  else if
+    (* RG 492: "in Missis festivis... de D. N. Iesu Christo Rege" -- the
+       last Sunday of October, {!Temporal_ef.christ_the_king}'s own named
+       slug. Confirmed against the Ordo (25 October 2026: "de Domino
+       Nostro Jesu Rege"). *)
+    slug = "ef-christ-the-king"
+  then Some Preface.Christ_the_king
+  else if
+    (* RG 490(a): "in festo Ascensionis Domini" -- the feast itself
+       (Easter+39), checked here by slug rather than folded into the
+       Ascension WINDOW below (which starts the day AFTER, Easter+40):
+       Ascension Day itself needs no window at all, its own slug already
+       identifies it uniquely. Confirmed against the Ordo (14 May 2026:
+       "Ascensionis, Communic pr."). *)
+    slug = "ef-ascension"
+  then Some Preface.Ascension
+  else if
+    (* RG 494(a): "in Missis de festo... Ss.mae Trinitatis" -- Trinity
+       Sunday itself (Easter+56), {!Temporal_ef}'s own named slug. Checked
+       ahead of 494(b)'s own WIDER de-Tempore grant (checked last below,
+       after every other window) for the same "specific propria before any
+       season fallback" reason every other title trigger is. Confirmed
+       against the Ordo (31 May 2026: "Trinit. II"). *)
+    slug = "ef-trinity"
+  then Some Preface.Trinity
+  else if
+    (* RG 496: see {!preface_st_joseph_slugs}'s own citation. *)
+    List.mem slug preface_st_joseph_slugs
+  then Some Preface.St_joseph
+  else if
+    (* RG 495: "in Missis festivis et votivis beatae Mariae Virginis" --
+       {!preface_bvm_slugs} (its own citation has the full account of why
+       it is wider than {!Precedence_ef.marian_slugs}) covers every
+       genuine Marian FEAST; [subject = Bvm] covers the one VOTIVE-shaped
+       office this engine models without a votive-Mass dimension of its
+       own (RG 78/91 entry 27, the Saturday Office of the BVM -- this
+       file's own header has the empirical confirmation, 3/10 January
+       2026). The Purification is deliberately ABSENT from both:
+       {!Precedence_ef.marian_slugs} already excludes it by name (its own
+       citation), and it never carries [subject = Bvm] (tagged [Lord]
+       instead, register §6.0) -- RG 495's own "praeterquam in festo
+       Purificationis" exclusion therefore holds by construction, not by
+       a guard written here.
+
+       [not (is_vigil slug)]: RG 495's own "festivis" reads "festum", not
+       "vigilia" -- the SAME RG 21/35 taxonomy distinction {!creed}'s own
+       RG 28-34 comment already makes ("a vigil is its OWN liturgical-day
+       category, distinct from 'festum'"), applied here for the first
+       time in THIS function because it is the first branch a vigil can
+       actually reach: {!Precedence_ef.marian_slugs} includes
+       "vigil-of-the-assumption" (that list's own citation), which without
+       this guard would wrongly claim [Bvm] for 14 August. FOUND, not
+       assumed: the FIUV Ordo's own 14 August entry reads "comm. I", not
+       "BMV" -- checked directly, the same as every other finding in this
+       branch's own history. Corroborates, from the opposite direction,
+       {!creed}'s own RG 28-34 comment: colitur's own Nativity WINDOW
+       below already excludes 24 December (the Nativity Vigil) by
+       construction (it starts at 25 December, never 24th), so this guard
+       makes the SAME "a vigil is not a festum" answer explicit here too,
+       rather than relying on a second, unrelated accident of a date
+       range to produce it. *)
+    (not (Precedence_ef.is_vigil slug))
+    && (List.mem slug preface_bvm_slugs || observed.Celebration.subject = Subject.Bvm)
+  then Some Preface.Bvm
+  else if
+    (* RG 484(a)'s own explicit Purification clause ("necnon in festo
+       Purificationis B. Mariae Virg.") -- 2 February, nowhere near the
+       Nativity's own Christmas-to-Epiphany calendar position, so this is
+       a standalone slug check, not part of the WINDOW test below (unlike
+       every other 484 trigger, which IS date-based). Checked here, after
+       the BVM check immediately above (which the Purification's own
+       [subject = Lord] tag never reaches) and before the Nativity window
+       (which its own actual date, 2 February, never reaches either) --
+       positioned with the rest of 484's own citations for readability,
+       not because anything below could otherwise pre-empt it. *)
+    slug = "purification-of-the-blessed-virgin-mary"
+  then Some Preface.Nativity
+  else if
+    (* RG 484(a)/(b) merged, per this file's own header: 25 December-1
+       January (the Nativity itself and its octave, propria) UNION 2-5
+       January (498(b)'s own explicit extra de-Tempore days) -- one
+       contiguous window, since both halves produce the identical
+       preface. Checked BEFORE Apostles (below) but AFTER every genuine
+       "divine mysteries/Persons" propria above, per this file's own
+       header (St John the Evangelist, 27 December, is the live witness:
+       Apostles would otherwise apply and does not). *)
+    (m = 12 && dd >= 25) || (m = 1 && dd <= 5)
+  then Some Preface.Nativity
+  else if
+    (* RG 497: "in Missis festivis et votivis Apostolorum et
+       Evangelistarum" -- {!preface_apostle_slugs} (its own citation has
+       the full account of why it is wider than {!creed_apostle_slugs}),
+       confirmed by this file's own header to produce the SAME preface
+       answer as the Ordo on every Apostle date outside the Nativity
+       octave: 11 June (Barnabas), 29-30 June (Peter & Paul, In
+       Commemoratione Pauli), 11 May (Philip & James, RG 484(b)'s own
+       witness against the Nativity window immediately above). Checked
+       AFTER the Nativity window specifically (RG 484(b)'s own narrower
+       carve-out), but before every OTHER season window below -- an
+       Apostle feast keeps his own preface inside Lent, Passiontide,
+       Paschaltide etc., where nothing narrows the exception the way
+       484(b) does. No [is_vigil] guard is needed here the way RG 495's
+       own branch above needs one: checked directly, no entry on
+       {!preface_apostle_slugs} is ever a vigil slug (every Apostle vigil
+       in the shipped data -- "vigil-of-sts-peter-paul" -- carries its own
+       distinct slug, absent from this list). *)
+    List.mem slug preface_apostle_slugs
+  then Some Preface.Apostles
+  else if
+    (* RG 485(a): "in Missis de festo Epiphaniae et de Commemoratione
+       Baptismatis D. N. Iesu Christi" -- the feast itself and its own
+       named commemoration (13 January, {!Precedence_ef}'s own
+       "commemoration-of-the-baptism-of-the-lord" -- {!creed}'s own 475(c)
+       comment already documents this entry's [subject = Lord] tag), both
+       checked by slug so 485(b)'s own WIDER window below need not repeat
+       them. *)
+    slug = "ef-epiphany" || slug = "commemoration-of-the-baptism-of-the-lord"
+  then Some Preface.Epiphany
+  else if
+    (* RG 485(b): "diebus a 7 ad 13 ianuarii" -- every OTHER Mass in this
+       window (Holy Family Sunday, an ordinary Time-after-Epiphany feria
+       or Sunday, a saint's feast with no propria of its own), confirmed
+       against the Ordo's own 11 January 2026 entry (Holy Family Sunday:
+       "de Epiphania. II", not a Holy-Family-specific preface -- this
+       engine has none to offer it anyway). Colitur's own Christmastide
+       season already spans 25 December-13 January (RG 72-73,
+       {!Vocab_ef.season}'s own citation), so this window is exactly its
+       OWN post-Epiphany tail; written as an explicit date range rather
+       than a season test only because the Nativity window above already
+       claims the season's FIRST half by date, not by season either, for
+       symmetry. *)
+    m = 1 && dd >= 6 && dd <= 13
+  then Some Preface.Epiphany
+  else if
+    (* RG 486(a)/(b) merged: Ash Wednesday (Easter-46) through the
+       Saturday before Passion Sunday I (Easter-15) inclusive -- every
+       Lenten feria/Sunday's own Mass (a), and every OTHER Mass in the
+       same span lacking a proper of its own (b). Confirmed against the
+       Ordo throughout (e.g. 18 February/19-20 February 2026: "Quadr.").
+       {!creed}'s own RG 23 comment already explains why Ash Wednesday
+       (feria I classis) reaches this branch on [observed]'s own terms
+       regardless of rank -- this function reads no rank at all here,
+       only the date. *)
+    n >= -46 && n <= -15
+  then Some Preface.Lent
+  else if
+    (* RG 487(a)/(b) merged: Passion Sunday I (Easter-14) through Holy
+       Thursday (Easter-3) inclusive -- (a)'s own "de tempore"/festive-
+       votive half extends through Holy Thursday itself (the Mass of the
+       Lord's Supper), (b)'s own narrower saint-Mass half stops one day
+       earlier (Holy Wednesday) but reaches no LIVE day this check does
+       not already cover identically (Holy Thursday is a feria I classis,
+       RG 23(b), so no saint's feast can ever occupy it -- {!creed}'s own
+       RG 23 citation). Confirmed against the Ordo throughout (22 March
+       2026, Passion Sunday: "de Sancta Cruce."; 2 April 2026, Holy
+       Thursday: "de Sancta Cruce, Communicantes..."). *)
+    n >= -14 && n <= -3
+  then Some Preface.Holy_cross
+  else if
+    (* RG 489(a)/(b) merged: the Easter Vigil Mass (Easter-1, on Holy
+       Saturday's own date) through the vigil of the Ascension (Easter+38)
+       inclusive. [n = -1] is this engine's own OVERLOADED representation
+       of "the Vigil Mass", not Holy Saturday's daytime (which has no Mass
+       of its own at all, unlike Good Friday's [n = -2] this function
+       excludes by name above) -- [gloria]'s own RG 431(c) comment already
+       establishes the same convention for the identical date, and RG
+       489(a) resolves the question on its own terms regardless: the
+       Paschal preface's window STARTS at the Vigil Mass, so [n = -1] is
+       correctly [Easter]. Confirmed against the Ordo throughout (5 April
+       2026, Easter Sunday: "Pasch."; 13 May 2026, the Ascension Vigil:
+       "Pasch. I"); the fixture prints nothing at all for Holy Saturday's
+       own daytime square (4 April 2026), corroborating rather than
+       contradicting this reading -- see test_fiuv_ordo.ml's own citation. *)
+    n >= -1 && n <= 38
+  then Some Preface.Easter
+  else if
+    (* RG 490(b): "a feria VI post Ascensionem usque ad feriam VI ante
+       vigiliam Pentecostes" -- the Friday after Ascension (Easter+40)
+       through the Friday before the Pentecost vigil (Easter+47)
+       inclusive; Ascension Day itself (Easter+39) is already handled by
+       its own slug check above, not repeated here. Confirmed against the
+       Ordo throughout (15-22 May 2026: "Ascensionis"). *)
+    n >= 40 && n <= 47
+  then Some Preface.Ascension
+  else if
+    (* RG 493(a)/(b) merged: the vigil of Pentecost (Easter+48) through
+       "subsequens sabbatum" (the FOLLOWING Saturday, i.e. the Ember
+       Saturday within the Octave of Pentecost, Easter+55) inclusive.
+       Confirmed against the Ordo throughout (23-24 May 2026, the vigil
+       and Pentecost itself: "de Spirito Sancto"; 30 May 2026, the Ember
+       Saturday: "de Spirito Sancto"). *)
+    n >= 48 && n <= 55
+  then Some Preface.Holy_spirit
+  else if
+    (* RG 494(b): "in dominicis Adventus, et in omnibus dominicis II
+       classis, extra tempus natalicium et paschale". Read off
+       [temporal]'s own season, NOT [observed]'s rank -- CORRECTED from an
+       earlier version of this branch that DID read [observed.rank] and
+       required it to equal [Class2], which is wrong for the identical RG
+       16(a) reason {!Precedence.rules.admit}'s own [~temporal] parameter
+       exists and [creed]'s own 237(b)/475(a) comments already give: a
+       feast that has WON the day can carry a different rank than the
+       Sunday it stands on. FOUND, not assumed: All Saints' Day (1
+       November), Class1, observed outright over an ordinary
+       Time-after-Pentecost Sunday it commemorates
+       ([+ef-time-after-pentecost-sunday-23]), reads "Trinit." in the
+       Ordo -- [observed.rank] there is [Class1], so the OLD guard wrongly
+       answered [Common]; there is no dedicated preface for All Saints
+       among RG 484-497's own fourteen, so RG 482's chain correctly falls
+       through to the SUNDAY's own de-Tempore grant regardless of which
+       rank actually won the day.
+
+       {!Temporal_ef.temporal}'s own [match s with Advent | Lent -> Class1
+       | _ -> Class2] means EVERY Sunday's own TEMPORAL identity is
+       [Class2] except in Advent and Lent -- so "in omnibus dominicis II
+       classis" and "in dominicis Adventus" collapse into ONE test, "any
+       Sunday outside Christmastide and Paschaltide" (Lent's own Sundays
+       need no explicit exclusion here: {!creed}'s own RG 23/Lent-window
+       reasoning already means every one of them is claimed by the LENT
+       window earlier in this very priority chain, provably unreachable
+       here, the same "checked, not merely assumed" position the previous
+       version of this comment already took for Christmastide/Paschaltide
+       -- confirmed by the SAME domain sweep in test_rubrics_ef.ml, which
+       still finds zero Christmastide/Paschaltide/Lent days reaching this
+       branch after this change). Confirmed against the Ordo throughout
+       (e.g. every Advent/Time-after-Epiphany/Septuagesima/Time-after-
+       Pentecost Sunday not otherwise claimed: "Trinit."), now including
+       All Saints' Day itself. *)
+    temporal.Temporal.weekday = Date.Sun
+    && temporal.Temporal.season <> Vocab_ef.Christmastide
+    && temporal.Temporal.season <> Vocab_ef.Paschaltide
+  then Some Preface.Trinity
+  else
+    (* RG 498: "in Missis quae praefatione propria carent, nec sumere
+       debent praefationem de Tempore" -- everything else: an ordinary
+       weekday feria outside every window above, a plain sanctoral saint
+       with no title of his own, an ordinary (non-Sunday, non-Class2, or
+       Christmastide/Paschaltide) day. Confirmed against the Ordo
+       throughout (the single most common value in the fixture, 188 of
+       360 comparable rows). *)
+    Some Preface.Common
diff --git a/lib/rites/rite_ef/rubrics_ef.mli b/lib/rites/rite_ef/rubrics_ef.mli
index 618af65..befbf72 100644
--- a/lib/rites/rite_ef/rubrics_ef.mli
+++ b/lib/rites/rite_ef/rubrics_ef.mli
@@ -68,3 +68,38 @@ val gloria :
   observed:Vocab_ef.rank Celebration.t ->
   date:Date.t ->
   bool
+
+(** RG 487(a)'s own two fixed-feast triggers for the Holy Cross preference
+    (the Exaltation of the Holy Cross, the Most Precious Blood) -- see the
+    .ml's own citation for what RG 487(a)'s further-named categories (the
+    Passion/Instruments of the Passion, the Most Holy Redeemer) are absent
+    from the shipped calendar entirely, not merely from this list. *)
+val preface_holy_cross_slugs : string list
+
+(** RG 496's own two St Joseph feasts in the shipped calendar. *)
+val preface_st_joseph_slugs : string list
+
+(** RG 497's own [Apostolorum et Evangelistarum] population -- WIDER than
+    {!creed_apostle_slugs} (no natalicium restriction); see the .ml's own
+    citation for the FIUV Ordo evidence and what was deliberately left
+    off, unwitnessed. *)
+val preface_apostle_slugs : string list
+
+(** RG 495's own [beatae Mariae Virginis] population -- WIDER than
+    {!Precedence_ef.marian_slugs} (a different, looser standard than that
+    list's own RG 112(d) oration-level one); see the .ml's own citation
+    for the FIUV Ordo evidence. *)
+val preface_bvm_slugs : string list
+
+(** Which preface is said at this day's Mass (RG 482-499). [None] both when
+    this engine resolves a day with no Mass at all (Good Friday) and -- for
+    a rite that has not implemented this function at all -- as the type's
+    own neutral value; see the .ml's own header for the rubric quoted in
+    full, the priority order every branch follows and why, and the FIUV
+    Ordo evidence that order rests on. Same three parameters as {!creed}/
+    {!te_deum}/{!gloria}, for the same reasons. *)
+val preface :
+  temporal:(Vocab_ef.season, Vocab_ef.rank) Temporal.t ->
+  observed:Vocab_ef.rank Celebration.t ->
+  date:Date.t ->
+  Preface.t option