Ppxlib_traverse_builtins.T
type 'a map = 'a -> 'a
type 'a iter = 'a -> unit
type ('a, 'acc) fold = 'a -> 'acc -> 'acc
type ('a, 'acc) fold_map = 'a -> 'acc -> 'a * 'acc
type ('ctx, 'a) map_with_context = 'ctx -> 'a -> 'a
type ('a, 'res) lift = 'a -> 'res