feat(kernel): a type for which Mass a day says

The lectionary's four-step chain already decides whether a day says its own
proper, its own slug's entry, the preceding Sunday's Mass or a Common, and
then discards that decision once the citations are out. An ordo needs to
print it.

2 files changed, 34 insertions, 0 deletions

diff --git a/lib/kernel/mass_formulary.ml b/lib/kernel/mass_formulary.ml
new file mode 100644
index 0000000..8151720
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+++ b/lib/kernel/mass_formulary.ml
@@ -0,0 +1,8 @@
+type source = Proper | Own_slug | Preceding_sunday | Common [@@deriving sexp]
+type t = { said : Slug.t; via : source } [@@deriving sexp]
+
+let source_to_string = function
+  | Proper -> "proper"
+  | Own_slug -> "own"
+  | Preceding_sunday -> "preceding-sunday"
+  | Common -> "common"
diff --git a/lib/kernel/mass_formulary.mli b/lib/kernel/mass_formulary.mli
new file mode 100644
index 0000000..caea3eb
--- /dev/null
+++ b/lib/kernel/mass_formulary.mli
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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 *)
+[@@deriving sexp]
+
+type t = { said : Slug.t; via : source } [@@deriving sexp]
+
+(** The slug whose Mass is said. For {!Proper} and {!Own_slug} this is the day's
+    own; for {!Preceding_sunday} it is that Sunday's TEMPORAL slug; for
+    {!Common} it is the Common's own id. *)
+
+val source_to_string : source -> string