(defun c-curve (length angle min-length)
   (cond (( < length min-length) (plot-line length angle))
         (t (c-curve (/ length (sqrt 2))
                     (+ angle (/ pi 4))
                     min-length)
            (c-curve (/ length (sqrt 2))
                     (- angle (/ pi 4))
                     min-length))))
;
(defun dragon-curve (length angle sign min-length)
   (cond (( < length min-length) (plot-line length angle)
         (t (dragon-curve (/ length (sqrt 2))
                          (+ angle (* sign (/ pi 4)))
                          +1
                          min-length)
         (dragon-curve (/ length (sqrt 2))
                       (- angle (* sign (/ pi 4)))
                       -1
                       min-length))))
;