docs: def xform(self, number, base):
          """
          Get a number as a string.
  
          :param number: a number
          :type number: list of int
          :param int base: the base in which this number is being represented
          :raises BasesValueError: if config is unsuitable for number
          """
          if self.CONFIG.use_letters:
              digits = \
                 self._UPPER_DIGITS if self.CONFIG.use_caps else \
                 self._LOWER_DIGITS
              return ''.join(digits[x] for x in number)
          separator = '' if base <= 10 else self.CONFIG.separator
          return separator.join(str(x) for x in number)