True when Property is a property of Var. These
properties are available during goal- and term-expansion. Defined
properties are below. Future versions are likely to provide more
properties, such as whether the variable is a singleton or whether the
variable is referenced in the remainder of the term. See also goal_expansion/2.
Bool has the value true if 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.
True when variable appears with the given name in the source.
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