val repr states how Int63.t is represented, i.e. as an int or an int64, and
can be used for building Int63 operations that behave differently depending on the
representation (e.g. see core_int63.ml).
type ('underlying_type, 'intermediate_type) t = | Int : (int, int) t |
| Int64 : (int64, Base__.Int63_emul.t) t |