Consider the following example, from OLabl 2.02:

# let ack ?:x [< 0 >] :y = x * y;;
val ack : ?x:int -> y:int -> int = <fun>
# let hack funarg = funarg x:4 y:5;;
val hack : (x:int -> y:int -> 'a) -> 'a = <fun>
# hack ack;;
Characters 5-8:
This expression has type ?x:int -> y:int -> int but is here used with
type
  x:int -> y:int -> 'a

This seems like a bug.  Intuitively, labels x: and ?x: should unify.
If not, I would appreciate a brief explanation as to why not...

Forcibly replacing the optional argument with the same label via fun
works just fine:

# hack (fun :x -> ack :x);;
- : int = 20

Thanks for any insights,
John