Make2.FnVarious function combinators lifted into the Kleisli category.
val id : 'a -> ('a, 'e) T2(M).tid x a monadic identity function
val nothing : unit -> (unit, 'e) T2(M).tnothing is a computation that does nothing.
non f returns a negation of the function f.
apply_n_times ~n f creates a chaing of computation of size n made from applications of the same function to its own result.