(defconstant +graph1+ 
  '((a  (b c))
    (b  (d))
    (c  (b d))
    (d  (c e))
    (e  (f))
    (f  (g)))
  "A directed graph, represented as an association list between nodes (A-F)
   and, for each node, a set of the states you can get to from that node."
)

(defun next-nodes (node graph)
  "Returns a set of the nodes you can get to in the graph
   directly from the given node, i.e. in one step.
  "
  ;; Fill in the body of this function
)

(defun next-nodes-set (nodeset graph)
  "Nodeset is a set of nodes.  This function returns a set of the 
   nodes you can get to in the graph directly (i.e. in one step)
   from any node in nodeset.
  "
  ;; Fill in the body of this function
)

(defun exactly-reachable-nodes (source graph numsteps)
  "Returns the nodes in graph that you can get to from source 
   in exactly numsteps steps.
  "
  ;; This one's free: don't change it!
  ;; Yes, I know I'm using setf.  But at least I'm only
  ;; using it to side-effect a local variable, not a global.
  ;; There are ways to write this function without using 
  ;; setf at all, but I'm being lazy.  So sue me... :-)
  ;;
  (let ((nodeset (list source)))
    (dotimes (i numsteps)
       (setf nodeset (next-nodes-set nodeset graph)))
    nodeset))

(defun exactly-reachablep (source target graph maxsteps)
  "Returns t if you can get from source to target in graph
   in exactly maxsteps steps, nil otherwise.
  "
  ;; Fill in the body of this function
)