def add_relation(self, source, destination):
if self.in_sources(source):<EOL><INDENT>if self.forward[source] != destination:<EOL><INDENT>raise ValueError("<STR_LIT>"<EOL>"<STR_LIT>")<EOL><DEDENT>else:<EOL><INDENT>raise ValueError("<STR_LIT>")<EOL><DEDENT><DEDENT>elif self.in_destinations(destination):<EOL><INDENT>raise ValueError("<STR_LIT>"<EOL>"<STR_LIT>")<EOL><DEDENT>else:<EOL><INDENT>self.forward[source] = destination<EOL>self.reverse[destination] = source<EOL><DEDENT>
Add new a relation to the bejection