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(x .--> K) = K
proof
  thus Sum(x .--> K) = card Union (x .--> [:K,{x}:]) by Th4
    .= card [:K,{x}:] by Th7
    .= card K by CARD_1:69
    .= K;
end;
