∀ {α : Type u} {β : α → Type v} {x : BEq α} {x_1 : Hashable α}
  {motive : Std.ExtDHashMap α β → Std.ExtDHashMap α β → Prop} (m₁ m₂ : Std.ExtDHashMap α β),
  (∀ (a b : Std.DHashMap α β), motive (Std.ExtDHashMap.mk a) (Std.ExtDHashMap.mk b)) → motive m₁ m₂