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
  for D being set, R being D-valued Relation
  for y being object st y in rng R holds y in D
proof
  let D be set, R be D-valued Relation;
  rng R c= D by Def17;
  hence thesis;
end;
