proposition: Lemma NS_log n: natp n -> natp (clog2 n).
proof: Proof. move => nN; case: (equal_or_not n \0c) => nz; first by rewrite nz clog0; fprops. by move:(clog_nz nN nz) => []. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(binomial div)