proposition: Lemma imset_union_incl {U T : Type} (s1 s2 : set U) (f : U -> T) : f @: (s1 :|: s2) \subset (f @: s1) :|: (f @: s2).
proof: Proof. firstorder. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)