∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {m₁ m₂ : Std.ExtHashSet α} [inst : EquivBEq α] [inst_1 : LawfulHashable α]
  {k : α} (not_mem : k ∉ m₂) {h' : k ∈ m₁ ∪ m₂}, (m₁ ∪ m₂).get k h' = m₁.get k ⋯