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+(** Which Mass a day actually says, and how that was decided.
+
+    A day does not always say its own Mass. A weekday with no proper of its own
+    resumes the preceding Sunday's; a saint with no proper says a Common. The
+    resolution already happens inside a rite's {!Rite.readings} -- this type is
+    what makes the answer visible instead of discarding it once the citations
+    have been extracted. An ordo prints it as "Mass of the 9th Sunday after
+    Pentecost".
+
+    Rite-agnostic by construction: it names a slug and a provenance, and
+    carries no rank, season or rubric vocabulary of any rite. *)
+
+type source =
+  | Proper  (** the observed celebration's own citations *)
+  | Own_slug  (** the lectionary's entry for the day's own slug *)
+  | Preceding_sunday  (** a weekday with no proper resumes the preceding Sunday *)
+  | Common  (** a saint's assigned Common *)
+  | Votive
+      (** a Mass said IN PLACE of the day's own office's Mass, the office
+          itself being kept unchanged -- RG 309(a): "in Ecclesia universa,
+          Missae quae pro sancta Maria in sabbato, iuxta temporum
+          diversitatem, in Missali assignantur", corroborated by RG 431(e)'s
+          own classification of that Mass as "Missa votiva IV classis ...
+          de B. Maria Virg." The office/Mass split this constructor exists
+          to name is general (any rite may say a votive Mass on a day whose
+          OFFICE is not itself votive), even though EF's only witness today
+          is the RG 78/309(a) Saturday Mass of Our Lady. *)
+[@@deriving sexp]
+
+type t = { said : Slug.t option; via : source } [@@deriving sexp]
+
+(** The slug whose CITATIONS were actually used to build the day's
+    readings, when the shipped data can name one.
+
+    [Some] for four of the five sources: {!Proper} and {!Own_slug} each
+    carry the day's own slug; {!Preceding_sunday} carries that Sunday's
+    TEMPORAL slug; {!Common} carries the Common's own id.
+
+    [None] for {!Votive}, and ONLY for {!Votive} (whole-branch review fix
+    round, celebrant-rubrics-phase1): RG 309(a)'s five seasonal "Missae de
+    sancta Maria in sabbato" carry no slug of their own anywhere in the
+    shipped data -- their citations come from a season-keyed function
+    ({!Rite_ef.Lectionary_ef.bvm_saturday_citations}), never from a
+    [Lectionary.find] against any slug, so there is genuinely no slug this
+    field could honestly report. An earlier version of this type set
+    [said] to the day's own OFFICE slug for {!Votive} too (RG 78's Office
+    of Our Lady, kept unchanged under the votive Mass) -- readable at the
+    call site as "this is the slug whose Mass is said", which is false for
+    exactly this one case: that slug's own [Lectionary] entry, if it has
+    one at all, is NOT what the day's citations came from. [None] says so
+    directly instead of silently mis-naming a Mass. The day's own OFFICE
+    is not lost by this change -- it is still on the very same
+    {!Liturgical_day.t} this formulary lives on, via [observed.slug],
+    which every caller already has in scope regardless of [via]; this
+    field does not need to duplicate it. *)
+
+val source_to_string : source -> string