reserve A,X,X1,X2,Y,Y1,Y2 for set, a,b,c,d,x,y,z for object;
reserve P,P1,P2,Q,R,S for Relation;

theorem
  field R = field(R~)
proof
  thus field R = rng(R~) \/ rng R by Th14
    .= field(R~) by Th14;
end;
