proposition: Lemma opred_le x: ordinalp x -> opred x <=o x.
proof: Proof. move => ox;split; [ exact: (OS_sup (Os_ordinal ox)) | exact | ]. move => t /setU_P [y ty yx]; exact:(ordinal_transitive ox yx ty). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)