Fact ler_wpeexpz2l x (x1 : 1 <= x) :
{in >= 0 &, {homo (exprz x) : x y / x <= y}}.
Fact ler_wneexpz2l x (x1 : 1 <= x) :
{in <= 0 &, {homo (exprz x) : x y / x <= y}}.

Lemma mulz_sg_eqN1 x y : (sgz x * sgz y == -1) = (x != 0) && (sgz x == - sgz y).