<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def count_trackbacks_handler(sender, **kwargs):
 """ Update Entry.trackback_count when a trackback was posted. """

    entry = kwargs['entry']
    entry.trackback_count = F('trackback_count') + 1
    entry.save(update_fields=['trackback_count'])