proposition: Lemma tuto_existsP : reflect (exists x, P x) [exists x, P x].
proof: Proof. by apply: (iffP pred0Pn); case=> x; exists x. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finset)