code: def _make_identifier(string):
          
          string = re.sub(r"[ \-+/\\*%&$£#@.,;:'" "?<>]", "_", string)
          if re.match(r"^\d", string):
              string = "a_{0}".format(string)
          return string
docs: Attempt to convert string into a valid identifier by replacing invalid characters with "_"s,
          and prefixing with "a_" if necessary.