strong connected Graph

Hallo Friends:
I want to define a strong connected graph with lisp:
(let ((graph '(a (b nil . nil) c)))
       (setf (second (second graph)) (second graph))
      (setf (rest (last (second graph))) (rest (rest graph)))
       graph)
but intepreter displays error messages
 *** - Lisp stack overflow. RESET
Why is this graph false?
By the way,how can I correct build a strong-connected graph with lisp 
and how can I such each atom in this graph?  

Best Regurd
Xu