code: def element(self, inp=None):
          
          if inp is None:
              return self.elements[0]
          elif inp in self.elements:
              return inp
          else:
              raise ValueError('cannot convert inp {} to element in {}'
                               ''.format(inp, self))
docs: Create a new element.
  
          For more specific control, use set[i].element() to pick which subset to
          use.