You can search for identifiers within the package.
in-package search v0.2.0
type ('k, 'res) args =
| [] : ('res, 'res) args
| :: : 'a arg * ('k, 'res) args -> ('a -> 'k, 'res) args
and 'v arg
val req : 'a value -> 'a arg
val opt : 'a value -> 'a option arg
val dft : 'a value -> 'a -> 'a arg
val map : ('k, 'a) args -> 'k -> 'a arg