(defun merge-variables (input &optional output)
  (if (null input)
    (return-from merge-variables output))
  (merge-variables
    (mapcan #'(lambda (x)
      (if (not (eq (caar input) (car x)))
          (list x) )
    ) input)
  (check-for-zero-exponential (list (car (car input))
    (apply '+ (mapcan #'(lambda (x)
      (if (eq (caar input) (car x))
         (cdr x))
     ) input))) output)
   )
)