∀ {α : Type u} {β : α → Type v} {m : Std.DHashMap.Raw α β} [inst : BEq α] [inst_1 : Hashable α] [inst_2 : LawfulBEq α],
  m.WF → ∀ {a : α} {fallback : β a}, a ∈ m → m.get? a = some (m.getD a fallback)