message: Comment pointing out where we use null_vertex.
diff: @@ -500,6 +500,7 @@ class Simplex_tree {
  * sh has children.*/
  template<class SimplexHandle>
  bool has_children(SimplexHandle sh) const {
  + // Here we rely on the root using null_vertex(), which cannot match any real vertex.
  return (sh->second.children()->parent() == sh->first);
  }
  