∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : Type v} {m₁ m₂ : Std.HashMap α β} [EquivBEq α] [LawfulHashable α]
  [inst : Inhabited α] {k : α}, (m₁ \ m₂).getKey! k = if k ∈ m₂ then default else m₁.getKey! k