A VT-constant is a constant (i.e., not a variable) that denotes a VT-component, or valid value for a VT parameter-slot.
(<=> (Vt-Constant ?Constant)
(And (Constant ?Constant)
(Value-Type ?Constant Denotation Value-Allowed-For-Vt-Term)
(Value-Cardinality ?Constant Denotation 1)))