A BinaryFunction is associative if bracketing has no effect on the value returned by the Function. More precisely, a Function ?FUNCTION is associative just in case (?FUNCTION ?INST1 (?FUNCTION ?INST2 ?INST3)) is equal to (?FUNCTION (?FUNCTION ?INST1 ?INST2) ?INST3), for all ?INST1, ?INST2, and ?INST3.
