correspondence between + and `add'`: that is, `lift'` takes any two-place
operation on integers and returns a version that takes arguments of type `int
option` instead, returning a result of `int option`. In other words, `lift'`
correspondence between + and `add'`: that is, `lift'` takes any two-place
operation on integers and returns a version that takes arguments of type `int
option` instead, returning a result of `int option`. In other words, `lift'`