diff -r 4e2581021300 -r 2f7ae34e1333 lemon/smart_graph.h
--- a/lemon/smart_graph.h	Thu Apr 24 11:56:44 2008 +0200
+++ b/lemon/smart_graph.h	Thu Apr 24 13:53:09 2008 +0100
@@ -115,6 +115,13 @@
     static Node nodeFromId(int id) { return Node(id);}
     static Arc arcFromId(int id) { return Arc(id);}
 
+    bool valid(Node n) const { 
+      return n._id >= 0 && n._id < static_cast<int>(nodes.size()); 
+    }
+    bool valid(Arc a) const { 
+      return a._id >= 0 && a._id < static_cast<int>(arcs.size()); 
+    }
+
     class Node {
       friend class SmartDigraphBase;
       friend class SmartDigraph;
@@ -261,6 +268,24 @@
     /// \sa reserveNode
     void reserveArc(int m) { arcs.reserve(m); };
 
+    /// \brief Node validity check
+    ///
+    /// This function gives back true if the given node is valid,
+    /// ie. it is a real node of the graph.  
+    ///
+    /// \warning A removed node (using Snapshot) could become valid again
+    /// when new nodes are added to the graph.
+    bool valid(Node n) const { return Parent::valid(n); }
+
+    /// \brief Arc validity check
+    ///
+    /// This function gives back true if the given arc is valid,
+    /// ie. it is a real arc of the graph.  
+    ///
+    /// \warning A removed arc (using Snapshot) could become valid again
+    /// when new arcs are added to the graph.
+    bool valid(Arc a) const { return Parent::valid(a); }
+
     ///Clear the digraph.
     
     ///Erase all the nodes and arcs from the digraph.
@@ -550,6 +575,16 @@
     static Arc arcFromId(int id) { return Arc(id);}
     static Edge edgeFromId(int id) { return Edge(id);}
 
+    bool valid(Node n) const { 
+      return n._id >= 0 && n._id < static_cast<int>(nodes.size()); 
+    }
+    bool valid(Arc a) const { 
+      return a._id >= 0 && a._id < static_cast<int>(arcs.size());
+    }
+    bool valid(Edge e) const { 
+      return e._id >= 0 && 2 * e._id < static_cast<int>(arcs.size()); 
+    }
+
     Node addNode() {     
       int n = nodes.size();
       nodes.push_back(NodeT());
@@ -642,6 +677,33 @@
       return Parent::addEdge(s, t); 
     }
 
+    /// \brief Node validity check
+    ///
+    /// This function gives back true if the given node is valid,
+    /// ie. it is a real node of the graph.  
+    ///
+    /// \warning A removed node (using Snapshot) could become valid again
+    /// when new nodes are added to the graph.
+    bool valid(Node n) const { return Parent::valid(n); }
+
+    /// \brief Arc validity check
+    ///
+    /// This function gives back true if the given arc is valid,
+    /// ie. it is a real arc of the graph.  
+    ///
+    /// \warning A removed arc (using Snapshot) could become valid again
+    /// when new edges are added to the graph.
+    bool valid(Arc a) const { return Parent::valid(a); }
+
+    /// \brief Edge validity check
+    ///
+    /// This function gives back true if the given edge is valid,
+    /// ie. it is a real edge of the graph.  
+    ///
+    /// \warning A removed edge (using Snapshot) could become valid again
+    /// when new edges are added to the graph.
+    bool valid(Edge e) const { return Parent::valid(e); }
+
     ///Clear the graph.
     
     ///Erase all the nodes and edges from the graph.