theorem zeqmeq1 (_n1 _n2 a b: nat): $ _n1 = _n2 -> (modZ(_n1): a = b <-> modZ(_n2): a = b) $;
_n1 = _n2 -> _n1 = _n2
_n1 = _n2 -> (modZ(_n1): a = b <-> modZ(_n2): a = b)