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trueif the variable is guaranteed to be unbound at entry of the goal, otherwise its value is false. This implies that the variable first appears in this goal or a previous appearance was in a negation (\+/1) or a different branch of a disjunction.
trueif the variable is a syntactic singleton in the term it appears in. Note that this tests that the variable appears exactly once in the term being expanded without making any claim on the syntax of the variable. Variables that appear only once in multiple branches are not singletons according to this property. Future implementations may improve on that.