Naf

Negation as failure (\+/1). Conceptually equivalent to

\+(G) :- call(G), !, fail.

in classic Prolog, but implemented directly as a it.unibo.tuprolog.solve.primitive.Primitive here, spawning an independent ConcurrentSolver (via subSolver()) to check whether its argument has at least one solution, rather than as a rule. Since it.unibo.tuprolog.solve.concurrent.stdlib.DefaultBuiltins registers this object under the very same \+/1 signature as the rule-based it.unibo.tuprolog.solve.concurrent.stdlib.rule.NegationAsFailure, and primitives are looked up before rules (see it.unibo.tuprolog.solve.concurrent.fsm.StatePrimitiveSelection), this primitive always wins and that rule pair is currently dead code (see the // TODO remove rule for \+ from default builtins comment next to this object's own registration in DefaultBuiltins).