An open variable is a type of variable indicated by a ? preceding its arbitrarily-chosen name, and which is not in the scope of any quantifier like #$forAll or #$thereExists — i.e. it is a variable that is not bound by a quantifier. Open variables often appear in queries where there is no explicit existential or universal quantification. In the antecedent of asserted rules, variables that appear open are always and implicitly universally quantified.
