theorem :: CARD_2:9

for A, B being Ordinal
for x1, x2 being object st x1 <> x2 holds
( A +^ B,[:A,{x1}:] \/ [:B,{x2}:] are_equipotent & card (A +^ B) = card ([:A,{x1}:] \/ [:B,{x2}:]) ) by Lm1;