<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, hl_group, line, col_start=0, col_end=-1, src_id=-1, async_=None, **kwargs):
 """Add a highlight to the buffer."""

        async_ = check_async(async_, kwargs, src_id != 0)
        return self.request('nvim_buf_add_highlight', src_id, hl_group,
                            line, col_start, col_end, async_=async_)