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Predicate //3
 +Free / :Lambda
 /(+Free, :Lambda, ?A1)
 /(+Free, :Lambda, ?A1, ?A2)
 /(+Free, :Lambda, ?A1, ?A2, ?A3)
 /(+Free, :Lambda, ?A1, ?A2, ?A3, ?A4)
 /(+Free, :Lambda, ?A1, ?A2, ?A3, ?A4, ?A5)
 /(+Free, :Lambda, ?A1, ?A2, ?A3, ?A4, ?A5, ?A6)
 /(+Free, :Lambda, ?A1, ?A2, ?A3, ?A4, ?A5, ?A6, ?A7)
Shorthand for Free/[]>>Lambda. This is the same as applying call/N on Lambda, except that only variables appearing in Free are bound by the call. For example 
p(1,a).
p(2,b).

?- {X}/p(X,Y).
X = 1;
X = 2.

This can in particularly be combined with bagof/3 and setof/3 to select particular variables to be concerned rather than using existential quantification (^/2) to exclude variables. For example, the two calls below are equivalent.

setof(X, Y^p(X,Y), Xs)
setof(X, {X}/p(X,_), Xs)
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