bipartiteness tests
  even cycle C6      : bipartite; side 0 = {0,2,4}, side 1 = {1,3,5}
  odd cycle C5       : NOT bipartite (edge 2-3 joins two vertices of the same side)
  path P4            : bipartite; side 0 = {0,2}, side 1 = {1,3}
  complete K_{2,3}   : bipartite; side 0 = {0,1}, side 1 = {2,3,4}
  K4 (complete, n=4) : NOT bipartite (edge 1-2 joins two vertices of the same side)
  chapter graph a..e : NOT bipartite (edge 2-3 joins two vertices of the same side)
  two disjoint edges : bipartite; side 0 = {0,2}, side 1 = {1,3}
