code: def _to_alpha(self, num):
          
          num = ''.join(c for c in text_type(num) if c in self._num_set)
          return num.translate(self._num_trans)
docs: Convert a Kölner Phonetik code from numeric to alphabetic.
  
          Parameters
          ----------
          num : str or int
              A numeric Kölner Phonetik representation
  
          Returns
          -------
          str
              An alphabetic representation of the same word
  
          Examples
          --------
          >>> pe = Koelner()
          >>> pe._to_alpha('862')
          'SNT'
          >>> pe._to_alpha('657')
          'NLR'
          >>> pe._to_alpha('86766')
          'SNRNN'