🚀 go-pugleaf

i have two questions about prologs which contain a freeze/2 predicate
such as sicstus prolog.

1) how can one delay a goal until _either_ of two variables are
instantiated?

something like this could be done by the following,

freeze(X,Y,G) :- freeze(X,G), freeze(Y,G).

but this has the nasty consequence that G is called twice.

note that 

freeze(X,freeze(Y,G))

does _not_ work since Y may instantiated before X, and i want G to be
called in that case, too.


2) how can freeze/2 be used to do co-routining?  does this require a
modification of what is normally taken to be co-routining?