(** 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