let U1, U2 be Universe; :: thesis: ( U1 in U2 or U1 = U2 or U2 in U1 )
A1: ( On U1 in On U2 or On U1 = On U2 or On U2 in On U1 ) by ORDINAL1:14;
A2: U2 = Rank (On U2) by Th50;
U1 = Rank (On U1) by Th50;
hence ( U1 in U2 or U1 = U2 or U2 in U1 ) by A1, A2, CLASSES1:36; :: thesis: verum