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+(** 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