///\brief A wrapper for composing a subgraph of a 

   ///\brief A wrapper for composing a subgraph of a 

   /// bidirected graph made from a directed one. 

   /// bidirected graph made from a directed one. 

   ///

   ///

   ///\warning Graph wrappers are in even more experimental state than the other

   ///\warning Graph wrappers are in even more experimental state than the other

   ///parts of the lib. Use them at you own risk.

   ///parts of the lib. Use them at you own risk.

   ///

   ///

   /// Suppose that for a directed graph $G=(V, A)$, 

   /// Suppose that for a directed graph $G=(V, A)$, 

   /// two bool valued maps on the edge-set, 

   /// two bool valued maps on the edge-set,