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.

def add_format(self, key, value=None):
        """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.
        """
        if key in self.FORMAT:
            return
        self.FORMAT.append(key)
        if value is not None:
            for call in self:
                call.data.setdefault(key, value)