theorem zleeq2 (m _n1 _n2: nat): $ _n1 = _n2 -> (m <=Z _n1 <-> m <=Z _n2) $;
_n1 = _n2 -> _n1 = _n2
_n1 = _n2 -> (m <=Z _n1 <-> m <=Z _n2)