Make.Mval return : 'a -> 'a treturn a injects a into the monadic type.
bind f a is the sequential composition of two actions, passing any value produced by a as argument to f.
compose g f is the Kleisli composition of f and g, passing the input to f, then binding the output to g.
( let* ) ma f is bind f ma. This is a binding operator, and it is used as let* a = ma in f a.