for L1, L2 being RelStr
for f being Function of L1,L2 st f is isomorphic holds
( ( the carrier of L1 <> {} implies the carrier of L2 <> {} ) & ( the carrier of L2 <> {} implies the carrier of L1 <> {} ) & ( the carrier of L2 <> {} or the carrier of L1 = {} ) & ( the carrier of L1 = {} implies the carrier of L2 = {} ) & ( the carrier of L2 = {} implies the carrier of L1 = {} ) )