An instance is a Boolean circuit given as a structure: its elements are gates, each possibly marked as a constant true, a constant false, a conjunction, a disjunction or a negation, some gates are marked as outputs, and two binary relations wire a gate to its left and right arguments, a negation reading its left argument. The values of the gates are derived inductively, on two rails: a constant has its value; a conjunction is true when a left and a right argument are true, and false when some argument is false; a disjunction dually; a negation is true when its argument is false and false when it is true. The derivation is a least fixed point, so a gate on a cycle, or with missing arguments, derives no value. The instance is a yes-instance of CVP when some output gate derives the value true; CVP is the decision problem of the structures isomorphic to such an instance.