proposition: Lemma eqn_simpl2 n n' n'': (n * n' + n + n' == n * n'' + n + n'') = (n' == n'').
proof: Proof. by rewrite addnAC (addnAC _ _ n'') eqn_add2r - ! mulSnr eqn_mul2l. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)