proposition: Lemma extsubsI (s1 s2 : {set 'I_N}) : s1 \subset s2 -> subseq (extract s1) (extract s2).
proof: Proof. Proof using. by move/subsetP=> Hincl; apply: extsubsIm=> i; exact: Hincl. Qed.
imports: mathcomp(all_ssreflect) mathcomp(perm) sorted tools subseq partition tableau Schensted congr plactic ordcast ordtype