code: def phonetic_i_umlaut(sound: Vowel) -> Vowel:
          
          if sound.is_equal(a):
              return ee
          elif sound.is_equal(a.lengthen()):
              return ee.lengthen()
          elif sound.is_equal(o):
              return oee
          elif sound.is_equal(o.lengthen()):
              return oee.lengthen()
          elif sound.is_equal(u):
              return y
          elif sound.is_equal(u.lengthen()):
              return y.lengthen()
          if sound.is_equal(DIPHTHONGS_IPA_class["au"]):
              return DIPHTHONGS_IPA_class["ey"]
docs: >>> umlaut_a = OldNorsePhonology.phonetic_i_umlaut(a)
          >>> umlaut_a.ipar
          'ɛ'
  
          >>> umlaut_au = OldNorsePhonology.phonetic_i_umlaut(DIPHTHONGS_IPA_class["au"])
          >>> umlaut_au.ipar
          'ɐy'
  
          :param sound:
          :return: