examples/ex-51.rkt
#lang racket
(require (planet wcy/mpost-wrapper))
(provide main)
(define (main)
  (let* ((u '1cm)
         (p (path (point u 0) (dir-spec 'up)  ".."
                  (point (op* -1 u) 0) (dir-spec 'down) "--"
                  'cycle)))
    (fill p #:withcolor (op* 0.8 'white))
    (draw p #:withpen (scale '1bp 'pencircle))
    ))