proposition: Lemma half_succ n : natp n -> chalf (csucc (csucc n)) = csucc (chalf n).
proof: Proof. move => nN; move: (NS_half nN) => hN; move: (NS_succ hN) => shN. case: (cdouble_halfV nN) => ->; rewrite - (double_succ hN). by rewrite ! even_half. by rewrite ! odd_half. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(binomial div)