∀ {α : Type u} [inst : BEq α] [inst_1 : Hashable α] {β : Type v} {m₁ m₂ : Std.HashMap.Raw α β} [EquivBEq α]
  [LawfulHashable α],
  m₁.WF → m₂.WF → ∀ {k : α} {fallback : β}, (m₁ ∪ m₂).getD k fallback = m₂.getD k (m₁.getD k fallback)