type ('a, 'b, 'c, 'd) writer
= 'a writer0 ‑> ('b, 'c, 'd) S2.writer
type ('a, 'b, 'c, 'd) reader
= 'a reader0 ‑> ('b, 'c, 'd) S2.reader
type ('a, 'b, 'c, 'd) t
= 'a t0 ‑> ('b, 'c, 'd) S2.t