proposition: Lemma fun_image_set1 f x: function f -> inc x (source f) -> Vfs f (singleton x) = singleton (Vf f x).
proof: Proof. move=> ff xsf. apply: set1_pr; first by apply /dirim_set1_P; Wtac. by move => z /dirim_set1_P jg; apply:Vf_pr. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)