proposition: Lemma leq_sub2l p m n : m <= n -> p - n <= p - m.
proof: Proof. rewrite -(leq_add2r (p - m)) leq_subLR. by apply: leq_trans; rewrite -leq_subLR. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype) BinNat Ring