allHeadVariablesInNonNegatedLiterals

Whether every variable occurring in this Clause's head also occurs in at least one of its nonNegatedBodyLiterals — the Datalog requirement that a rule may not "invent" head variables that are never bound by a positive body goal. A it.unibo.tuprolog.core.Fact (empty body) vacuously satisfies this only if its head has no variables at all, since there are no non-negated body literals to bind them.