theorem zeqmeq2 (n _a1 _a2 b: nat): $ _a1 = _a2 -> (modZ(n): _a1 = b <-> modZ(n): _a2 = b) $;
_a1 = _a2 -> _a1 = _a2
_a1 = _a2 -> (modZ(n): _a1 = b <-> modZ(n): _a2 = b)