theorem :: E_SIEC:16

for N being e_net holds
( ( the entrance of N \ (id the carrier of N)) * ( the entrance of N \ (id the carrier of N)) = {} & ( the escape of N \ (id the carrier of N)) * ( the escape of N \ (id the carrier of N)) = {} & ( the entrance of N \ (id the carrier of N)) * ( the escape of N \ (id the carrier of N)) = {} & ( the escape of N \ (id the carrier of N)) * ( the entrance of N \ (id the carrier of N)) = {} )