theorem Set.mem_of_mem_of_subset
theorem Set.mem_of_mem_of_subset: ∀ {α : Type u} {x : α} {s t : Set α}, x ∈ s → s ⊆ t → x ∈ t
theorem Set.mem_of_mem_of_subset
theorem Set.mem_of_mem_of_subset: ∀ {α : Type u} {x : α} {s t : Set α}, x ∈ s → s ⊆ t → x ∈ t