proposition: Lemma gcdnC : commutative gcdn.
proof: Proof. move=> m n; wlog lt_nm: m n / n < m by have [? ->|? <-|-> //] := ltngtP n m. by rewrite gcdnE -[in m == 0](ltn_predK lt_nm) modn_small. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq)