code: def add_format(self, key, value=None):
          
          if key in self.FORMAT:
              return
          self.FORMAT.append(key)
          if value is not None:
              for call in self:
                  call.data.setdefault(key, value)
docs: Add an entry to format
  
          The record's calls ``data[key]`` will be set to ``value`` if not yet
          set and value is not ``None``.  If key is already in FORMAT then
          nothing is done.