docs: def placeholder(type_):
      """Returns the EmptyVal instance for the given type"""
      typetuple = type_ if isinstance(type_, tuple) else (type_,)
      if any in typetuple:
          typetuple = any
      if typetuple not in EMPTY_VALS:
          EMPTY_VALS[typetuple] = EmptyVal(typetuple)
      return EMPTY_VALS[typetuple]