proposition: Lemma ozero_dichot x: ordinalp x -> x = \0o \/ \0o <o x.
proof: Proof. move => ox; case: (equal_or_not x \0o) => xe; first by left. by right; apply: ord_ne0_pos. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)