Monad.COREBasis operations.
include Indexed_monad.WITH_RETURN with type ('a, 'index) t := 'a tinclude Indexed_monad.WITH_RETURN with type ('a, 'index) t := 'a tinclude Indexed_monad.WITH_RETURN with type ('a, 'index) t := 'a tval return : 'a -> 'a tLift a value into a t.