docs: def Format(self, format_string, rdf):
      """Apply string formatting templates to rdf data.
  
      Uses some heuristics to coerce rdf values into a form compatible with string
      formatter rules. Repeated items are condensed into a single comma separated
      list. Unlike regular string.Formatter operations, we use objectfilter
      expansion to fully acquire the target attribute in one pass, rather than
      recursing down each element of the attribute tree.
  
      Args:
        format_string: A format string specification.
        rdf: The rdf value to be formatted.
  
      Returns:
        A string of formatted data.
      """
      result = []
      for literal_text, field_name, _, _ in self.parse(format_string):
        # output the literal text
        if literal_text:
          result.append(literal_text)
        # if there's a field, output it
        if field_name is not None:
          rslts = []
          objs = self.expander(rdf, field_name)
          for o in objs:
            rslts.extend(self.FanOut(o))
          # format the objects and append to the result
          result.append(",".join(rslts))
      return "".join(result)