`w`. But that's not the right result, because `w` itself has been
mapped onto 2.
+## Baby monads
+
+(Depends on lecture notes for safe division by zero.)
+
+Write a function `lift'` that generalized the 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'`
+will have type:
+
+ (int -> int -> int) -> (int option) -> (int option) -> (int option)
+
+so that `lift' (+) (Some 3) (Some 4)` will evalute to `Some 7`.
+Don't worry about why you need to put `+` inside of parentheses.
+You should make use of `bind'` in your definition of `lift'`:
+
+ let bind' (u: int option) (f: int -> (int option)) =
+ match u with None -> None | Some x -> f x;;