define counter-clockwise point-x point-x point-y point-y point-y point

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(define (counter-clockwise p q r)
(- (* (- (point-x q) (point-x p))
(- (point-y r) (point-y p)))
(* (- (point-y q) (point-y p))
(- (point-x r) (point-x p)))))