void insert(box2d<double> const& box, mapnik::value_unicode_string const& text)
 {
     if (tree_.extent().intersects(box))
     {
         tree_.insert(label(box, text), box);
     }
 }
 void insert(box2d<double> const& box)
 {
     if (tree_.extent().intersects(box))
     {
         tree_.insert(label(box), box);
     }
 }
 box2d<double> const& extent() const
 {
     return tree_.extent();
 }