theorem Set.mem_preimage._simp_1
theorem Set.mem_preimage._simp_1: ∀ {α : Type u} {β : Type v} {f : α → β} {s : zpg9xh52.Set β} {a : α}, Membership.mem (zpg9xh52.Set.preimage f s) a = Membership.mem s (f a)
theorem Set.mem_preimage._simp_1
theorem Set.mem_preimage._simp_1: ∀ {α : Type u} {β : Type v} {f : α → β} {s : zpg9xh52.Set β} {a : α}, Membership.mem (zpg9xh52.Set.preimage f s) a = Membership.mem s (f a)