theorem Th2: :: EXCHSORT:2

for a, b, c, d being Ordinal st a in b & c in d & not ( c <> a & c <> b & d <> a & d <> b ) & not ( c in a & d = a ) & not ( c in a & d = b ) & not ( c = a & d in b ) & not ( c = a & d = b ) & not ( c = a & b in d ) & not ( a in c & d = b ) holds
( c = b & b in d )