code: def convert(self, string):
          
          if self == self.__class__.TITLE:
              return capitalize(string)
          if self == self.__class__.UPPER:
              return string.upper()
          if self == self.__class__.LOWER:
              return string.lower()
          return string
docs: Return a copy of string converted to case.
  
          Parameters
          ----------
          string : `str`
  
          Returns
          -------
          `str`
  
          Examples
          --------
          >>> CharCase.LOWER.convert('sTr InG')
          'str ing'
          >>> CharCase.UPPER.convert('sTr InG')
          'STR ING'
          >>> CharCase.TITLE.convert('sTr InG')
          'Str ing'
          >>> CharCase.PRESERVE.convert('sTr InG')
          'sTr InG'