code: def orthographic_u_umlaut(sound: str) -> str:
          
          if sound in OldNorsePhonology.U_UMLAUT:
              return OldNorsePhonology.U_UMLAUT[sound]
          else:
              return sound
docs: >>> OldNorsePhonology.orthographic_u_umlaut("a")
          'ö'
          >>> OldNorsePhonology.orthographic_u_umlaut("e")
          'e'
  
          :param sound:
          :return: