undirected graphs
  a-e graph of the chapter : has a cycle  (back edge c-a)
  tree on 5 vertices      : acyclic
  forest of two paths     : acyclic
  triangle + isolate      : has a cycle

directed graphs
  DFS example of the chapter: has a directed cycle  (back edge x->v)
  dressing DAG              : is a DAG
  dressing DAG + jacket->shirt: has a directed cycle
