∀ {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 fallback : α},
  (m₁ \ m₂).getKeyD k fallback = if k ∈ m₂ then fallback else m₁.getKeyD k fallback