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