theorem Set.mem_insert_of_mem
theorem Set.mem_insert_of_mem: ∀ {α : Type u} {a : α} {s : Set α} (b : α), a ∈ s → a ∈ insert b s
theorem Set.mem_insert_of_mem
theorem Set.mem_insert_of_mem: ∀ {α : Type u} {a : α} {s : Set α} (b : α), a ∈ s → a ∈ insert b s