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;

theorem
  pi({},x) = {}
proof
  set y = the Element of pi({},x);
  assume not thesis;
  then ex f st f in {} & y = f.x by Def6;
  hence contradiction;
end;
