Index: src/work/alpar/emptygraph.h
===================================================================
--- src/work/alpar/emptygraph.h	(revision 55)
+++ src/work/alpar/emptygraph.h	(revision 56)
@@ -13,38 +13,38 @@
   class NodeIt {};
     
-  NodeIt &getFirst(NodeIt &);
-  InEdgeIt &getFirst(InEdgeIt &,const NodeIt &);
-  OutEdgeIt &getFirst(OutEdgeIt &,const NodeIt &);
-  SymEdgeIt &getFirst(SymEdgeIt &,const NodeIt &);
-  EachEdgeIt &getFirst(EachEdgeIt &);
+  NodeIt &getFirst(NodeIt &) const;
+  InEdgeIt &getFirst(InEdgeIt &,const NodeIt) const;
+  OutEdgeIt &getFirst(OutEdgeIt &,const NodeIt) const;
+  SymEdgeIt &getFirst(SymEdgeIt &,const NodeIt) const;
+  EachEdgeIt &getFirst(EachEdgeIt &) const;
 
-  NodeIt next(const NodeIt &);
-  InEdgeIt next(const InEdgeIt &);
-  OutEdgeIt next(const OutEdgeIt &);
-  SymEdgeIt next(const SymEdgeIt &);
-  EachEdgeIt next(const EachEdgeIt &);
+  NodeIt next(const NodeIt) const;
+  InEdgeIt next(const InEdgeIt) const;
+  OutEdgeIt next(const OutEdgeIt) const;
+  SymEdgeIt next(const SymEdgeIt) const;
+  EachEdgeIt next(const EachEdgeIt) const;
 
-  NodeIt &goNext(const NodeIt &);
-  InEdgeIt &goNext(const InEdgeIt &);
-  OutEdgeIt &goNext(const OutEdgeIt &);
-  SymEdgeIt &goNext(const SymEdgeIt &);
-  EachEdgeIt &goNext(const EachEdgeIt &);
+  NodeIt &goNext(NodeIt &) const;
+  InEdgeIt &goNext(InEdgeIt &) const;
+  OutEdgeIt &goNext(OutEdgeIt &) const;
+  SymEdgeIt &goNext(SymEdgeIt &) const;
+  EachEdgeIt &goNext(EachEdgeIt &) const;
 
-  NodeIt head(const EdgeIt &);
-  NodeIt tail(const EdgeIt &);
+  NodeIt head(const EdgeIt) const;
+  NodeIt tail(const EdgeIt) const;
   
-  NodeIt aNode(const InEdgeIt &);
-  NodeIt aNode(const OutEdgeIt &);
-  NodeIt aNode(const SymEdgeIt &);
+  NodeIt aNode(const InEdgeIt) const;
+  NodeIt aNode(const OutEdgeIt) const;
+  NodeIt aNode(const SymEdgeIt) const;
 
-  NodeIt bNode(const InEdgeIt &);
-  NodeIt bNode(const OutEdgeIt &);
-  NodeIt bNode(const SymEdgeIt &);
+  NodeIt bNode(const InEdgeIt) const;
+  NodeIt bNode(const OutEdgeIt) const;
+  NodeIt bNode(const SymEdgeIt) const;
 
-  bool valid(const NodeIt &);
-  bool valid(const EdgeIt &);
+  bool valid(const NodeIt) const;
+  bool valid(const EdgeIt) const;
 
-  void setInvalid(const NodeIt &);
-  void setInvalid(const EdgeIt &);
+  void setInvalid(NodeIt &) const;
+  void setInvalid(EdgeIt &) const;
   
   NodeIt addNode();