(define c-curve
  (lambda (x0 y0 x1 y1 level)
    (if (= level 0)
        (line x0 y0 x1 y1)

        (let ((xmid (/ (+ x0 x1) 2))
              (ymid (/ (+ y0 y1) 2))
              (dx (- x1 x0))
              (dy (- y1 y0)))
          (let ((xa (- xmid (/ dy 2)))
                (ya (+ ymid (/ dx 2))))

            ... )))))