code: def file_and_line(self):
          
          ret = "%s:%d" % (self.source_file.path, self.lineno)
          if self.from_source_file:
              ret += " (%s:%d)" % (self.from_source_file.path, self.from_lineno)
          return ret
docs: Return the filename and line number where this rule originally
          appears, in the form "foo.scss:3".  Used for error messages.