∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : α → Type v} {m : Std.ExtDHashMap α β} [inst : LawfulBEq α] {k a : α}
  {fallback : β a}, (m.erase k).getD a fallback = if (k == a) = true then fallback else m.getD a fallback