Parameter Merge.Key

type 'a t
val sexp_of_t : ('a -> Sexplib0.Sexp.t) -> 'a t -> Sexplib0.Sexp.t
val type_id : 'a t -> 'a Base.Type_equal.Id.t

For correct behavior of the map, type_id must return the same Type_equal.Id on different calls on the same input.