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"Y c= R"rng R
proof
  R"Y c= dom R by Th124;
  hence thesis by Th126;
end;
