Optional arguments in functions???

Hi everyone,

I'm experiencing some issues with optional arguments passed to functions in
Ocaml.  For example, let's say I want to write a function like this:

let multiply x y ?z(z = 1) = x * y * z ;;

This doesn't work in utop.  What I'm trying to do is this.  If two
arguments are passed to the function, then just return the product of those
two arguments.  ( Because x * y * 1 is always the same as x * y. )  But if
three arguments are passed, then return the product of all three integers.
What am I doing wrong?  There must be a way to do this.  Thanks for sharing!

Best,

Douglas.