How to make this neighbor function?

I have this code:

(defparameter fc #\F)
(defparameter bc #\B)
(defparamete开发者_Python百科r gap #\G)

(defun solp (seq)
    (if (eql fc (car seq))
        (not (if (listp (cdr seq))
                 (find bc (cdr seq))
                 (eql seq bc)))
        (solp (cdr seq))))

(defun heuristic (seq &optional (f 0)) 
    (if (eql nil seq) 
        0   
        (if (eql bc (car seq))
            (+ f (heuristic (cdr seq) f)) 
            (heuristic (cdr seq) (+ f 1)))))

(defun genneighbors (seq)

    ;seq == (fc fc gap bc bc) ===> neighbors == ( (gap fc fc bc bc)
    ;                                              (fc gap fc bc bc)
    ;                                               (fc fc bc gap)
    ;                                               (fc fc bc bc gap) )
    ;I can't figure out how to do this
)

I can't figure out how to code the genneighbors function. How do I access an element (2 | 1) slots before gap How to generate all four possible neighbors? Can someone give me some pointers?

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All the functions from the CLHS chapters conses and sequences apply to lists.

Also note that it is good programming style (for various reasons) to name global variables like this: *bc*, *fc* and *gap*.

Also note that in newer code one often uses FIRST instead of CAR and REST instead of CDR.