Racklog provides special predicates for probing logic variables, without risking their getting bound.
(%== X Y)
succeeds if X and Y are identical objects. This is not quite the unification predicate %=, for %== doesn’t touch unbound objects the way %= does. Eg, %== will not equate an unbound logic variable with a bound one, nor will it equate two unbound logic variables unless they are the same variable.
succeeds if X isn’t completely bound —
Racklog lets the user protect a term with variables from unification by allowing that term to be treated as a (completely) bound object. The predicates provided for this purpose are %freeze, %melt, %melt-new, and %copy.
(%freeze S F)
unifies F to the frozen version of S. Any lack of bindings in S are preserved no matter how much you toss F about.
(%melt F S)
retrieves the object frozen in F into S.
(%melt-new F S)
is similar to %melt, except that when S is made, the unbound variables in F are replaced by brand-new unbound variables.
(%copy S C)