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
  Sum({} --> K) = 0
proof dom ({} --> K) = {};
  then dom disjoin ({} --> K) = {} by Def3;
  hence thesis by CARD_1:27,RELAT_1:42,ZFMISC_1:2;
end;
