proposition: Lemma card_fseq (T : choiceType) (s : seq T) : #|` [fset x in s] | = size (undup s).
proof: Proof. by rewrite card_imfset. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)