1) You have to implement the functions MAX-PLAYER and
MIN-PLAYER. Each takes three arguments: 

     *  the tree to be searched,
     *  an alpha value, and 
     *  a beta value, 

That is, 

(defun MAX-PLAYER (Tree Alpha Beta)
    . . . 
)

(defun MIN-PLAYER (Tree Alpha Beta)
    . . . 
)


The return value of these functions is the MINIMAX value
of the root of the Tree given as input.


2) The command defvar is used to declare a global variable 
and assign an initial value to it. This command is used in 
the specification of homework 4 to declare the variable 
*Counter*:

    (defvar *Counter* 0)

and the sample input trees:

    (defvar tree22 '((1 2) (3 4)))
   
    (defvar tree23 ...)

    ...

3) The variable counter is intended to measure the savings of 
the Alpha-Beta pruning algorithm. 

   If no pruning is possible (or if the pruning mechanism is
disabled) during the search, then after the search is performed, 
the variable *Counter* should be equal to the number of leaves 
in the tree. 

   However, in most inputs the Alpha-Beta pruning algorithm
will have many opportunities to prune the search. Thus, after
the execution of the search the variable *Counter* will be 
smaller than the number of leaves on the tree.

   For example, in the sample input tree23 (a tree with uniform
braching factor 2 and depth 3) we have 8 leaf nodes. Thus, after 
running a search algorithm which does not do any pruning, the 
value of counter should be 8. On the other hand, running Alpha-Beta 
pruning on the same input sets the variable *Counter* to 6. 

   The only thing you have to do to implement this counting
mechanism is to include the global variable declaration in your
file:

    (defvar *Counter* 0)

and every time the search reaches a leaf node you have to increment
the counter using the command:

    (setq *Counter* (+ *Counter* 1)

   When testing your program, I'll make sure to always reset 
*Counter* to zero before starting the search on my test input.
Note that you DON'T have to return of *Counter*. Since it is
a global variable I can always check it after the search finishes.