(mapcar #'print
  (list
    (next-nodes 'c +graph1+)
    (next-nodes-set '(c) +graph1+)
    (next-nodes-set '(a c) +graph1+)
    (next-nodes-set '(g) +graph1+)
    (next-nodes-set nil +graph1+)
    (exactly-reachable-nodes 'a +graph1+ 0)
    (exactly-reachable-nodes 'a +graph1+ 3)
    (exactly-reachable-nodes 'e +graph1+ 3)
    (mapcar #'(lambda (x) (exactly-reachable-nodes x +graph1+ 3)) '(a b c d e f g h))
    (exactly-reachablep 'a 'b +graph1+ 3)
    (exactly-reachablep 'a 'd +graph1+ 3)))