(defun percentage-lineariser (x)
  "Converts a percentage form into a more linear format. For example:
  4/5 ->  80% -> -0.25
  3/2 -> 150% ->  0.5
  2/1 -> 200% ->  1.0
  1/2 ->  50% -> -1.0
This format is easier to work with difference-judging strategies such as
EXPONENTIAL-MOVING-AVERAGE-STRATEGY. This function is undefined for negative
and zero values."
  (assert (plusp x) (x)
          "Percentage must be positive")
  (cond
    ((> x 1) (1- x))
    ((= x 1) 0)
    ((< x 1) (- 1 (/ x)))))