code: def add_relation(self, source, destination):
          
          if self.in_sources(source):
              if self.forward[source] != destination:
                  raise ValueError("Source is already in use. Destination does "
                                   "not match.")
              else:
                  raise ValueError("Source-Destination relation already exists.")
          elif self.in_destinations(destination):
              raise ValueError("Destination is already in use. Source does not "
                               "match.")
          else:
              self.forward[source] = destination
              self.reverse[destination] = source
docs: Add new a relation to the bejection