proposition: Lemma odd_dichot n: n = (n./2).*2.+1 \/ n = (n./2).*2.
proof: Proof. by rewrite -{1 3} (odd_double_half n);case: odd; [left | right]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq path fintype div bigop binomial) mathcomp(prime finset ssralg ssrnum ssrint)