proposition: Lemma osum2_a_ab a b: ordinalp a -> ordinalp b -> a +o b = a -> b = \0o.
proof: Proof. move => oa ob; rewrite - {2}(osum0r oa); apply: osum2_simpl; fprops. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)