code: def int2letters(x, alphabet):
      
      base = len(alphabet)
      if x < 0:
          raise ValueError('Only non-negative numbers are supported. Encounterd %s' % x)
      letters = []
      quotient = x
      while quotient >= 0:
          quotient, remainder = divmod(quotient, base)
          quotient -= 1
          letters.append(alphabet[remainder])
      letters.reverse()
      return ''.join(letters)
docs: Return the alphabet representation of a non-negative integer x.
      For example, with alphabet=['a','b']
      0 -> 'a'
      1 -> 'b'
      2 -> 'aa'
      3 -> 'ab'
      4 -> 'ba'
  
      Modified from:
      http://stackoverflow.com/questions/2267362/convert-integer-to-a-string-in-a-given-numeric-base-in-python