OK... It was un-runnable pseudo-code anyway; and which as function expects single argument; and, as a method, should be called on a mask without other arguments, right?

Even then, I'm confused, wouldn't you get exactly same result (i.e. $intersect) by simply and-ing ("&") the 2 masks and calling which on it? If example was just simplified for laymen to this tautology, I certainly expect no explanation, sorry to bother.