CGAL users discussion list

I write a code and within it I want to check whether it is simple or not:

  Rational               x, y;
  std::list<Rat_point_2> vertices;

  for (j = 0; j < n_vertices; j++)
  {
    ifile >> x >> y;
    vertices.push_back (Rat_point_2 (x, y));
  }

  Rat_polygon_2        pgn (vertices.begin(), vertices.end());

  if (! pgn.is_simple())
  {
    std::cerr << "Error - Polygon no. " << i + 1
              << " is not simple." << std::endl;
    return (false);
  }

for a polygon like:
2/1 5/1
1.5/1 3.5/1
3/1 3/1
3.5/1 4/1

the program found it is not simple!!!, I checked the usage of is_simple() 
with the one in the example\polygon
and they are similar.

but for a polygon like:
1/1 1/1
1/1 2/1
2/1 2/1
2/1 1/1
the program finds it as simple. 

what checks should I do more?