<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_highlight(self, field, fragment_size=None, number_of_fragments=None, fragment_offset=None, type=None):
 """Add a highlight field. The Search object will be returned, so calls to this can be chained. """

        if self._highlight is None:
            self._highlight = HighLighter("<b>", "</b>")
        self._highlight.add_field(field, fragment_size, number_of_fragments, fragment_offset, type=type)
        return self