I've not got time yet to write this up formally, but.... There's no difference between calling a sub PMC with parameters and invoking a return continuation with return values. The return values for that continuation are just sent as parameters, as if you were calling it as a function.
Yes, that means that if you do: $return_cont->(1, 2, 3); it's the same as: return (1,2,3); assuming $return_cont has your return continuation in it. If programs want to specify the types of the return values they can attach a prototype property to their return continuation, which can be queried the same (and yes, I know its unspecified) way that you can do it if you have a sub PMC. I'll update the PDD as soon as I can get time and some working brain cells. Dan