theorem ::#Th77:
rng f1 /\ rng f2 = {} & b1<>b2 & rng f1 c= dom f2 & rng f2 c= dom f1
implies [:rng f1,{b2}:]\/[:rng f2,{b1}:] c=
((f1\/f2) >*> ([:rng f1,{b2}:]\/[:rng f2,{b1}:])) >*> ((b1,b2)-->(b2,b1))
by Lm68;
