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
  R.:X c= R.:(dom R)
proof
  R.:X c= rng R by Th105;
  hence thesis by Th107;
end;
