reserve A,B,C for Ordinal,
  K,L,M,N for Cardinal,
  x,y,y1,y2,z,u for object,X,Y,Z,Z1,Z2 for set,
  n for Nat,
  f,f1,g,h for Function,
  Q,R for Relation;
reserve ff for Cardinal-Function;
reserve F,G for Cardinal-Function;

theorem
  F c= G implies Sum F c= Sum G
proof
  assume F c= G;
  then disjoin F c= disjoin G by Th23;
  hence thesis by Th24,CARD_1:11;
end;
