∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : α → Type v} {m₁ m₂ : Std.ExtDHashMap α β} [inst : LawfulBEq α]
  {k : α} {fallback : β k}, (m₁ \ m₂).getD k fallback = if k ∈ m₂ then fallback else m₁.getD k fallback