code: def build_chvatal_graph():
      
      # The easiest way to build the Chvatal graph is to start
      # with C12 and add the additional 12 edges
      graph = build_cycle_graph(12)
  
      edge_tpls = [
          (1,7), (1,9), (2,5), (2,11),
          (3,7), (3,9), (4,10), (4,12),
          (5,8), (6,10), (6,12), (8,11),
      ]
  
      for i, j in edge_tpls:
          graph.new_edge(i, j)
  
      return graph
docs: Makes a new Chvatal graph.
         Ref: http://mathworld.wolfram.com/ChvatalGraph.html