<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_rel(self, source_id, target_id, rel):
        ''' Cache aware add_rel '''
        if (source_id, target_id) not in self.rel_cache:
            self.workbench.add_rel(source_id, target_id, rel)
            self.rel_cache.add((source_id, target_id))