by move/ltnW->.
rewrite leq_eqVlt; case/orP; first by move/eqP->.
by move/modp_small; rewrite rpq => h; move: nq0; rewrite h eqxx.
Qed.

Lemma eqpP m n :
reflect (exists2 c12, (c12.1 != 0) && (c12.2 != 0) & c12.1 *: m = c12.2 *: n)
(m %= n).