code: def phonetic_u_umlaut(sound: Vowel) -> Vowel:
          
          if sound.is_equal(a):
              return oee  # or oe
          elif sound.is_equal(o):
              return u
          else:
              return sound
docs: >>> umlaut_a = OldNorsePhonology.phonetic_u_umlaut(a)
          >>> umlaut_a.ipar
          'ø'
  
          >>> umlaut_o = OldNorsePhonology.phonetic_u_umlaut(o)
          >>> umlaut_o.ipar
          'u'
  
          >>> umlaut_e = OldNorsePhonology.phonetic_u_umlaut(e)
          >>> umlaut_e.ipar
          'e'
  
  
          :param sound: instance of Vowel
          :return: