#(define (make-box thick padding filled fill-color open-on-left open-on-right xext yext)
   (let* ((xext (interval-widen xext padding))
          (yext (interval-widen yext padding)))
     (ly:stencil-add
      (if filled
          (ly:make-stencil (list 'color fill-color
                             (list 'round-filled-box
                               (- (- (car xext) thick)) (+ (cdr xext) thick)
                               (- (car yext)) (cdr yext)
                               0.0)
                             xext yext))
          empty-stencil)
      (if (> thick 0)
          (make-filled-box-stencil
           (cons (- (car xext) thick) (+ (cdr xext) thick))
           (cons (- (car yext) thick) (car yext)))
          empty-stencil)
      (if (> thick 0)
          (make-filled-box-stencil
           (cons (- (car xext) thick) (+ (cdr xext) thick))
           (cons (cdr yext) (+ (cdr yext) thick)))
          empty-stencil)
      (if (and (not open-on-right) (> thick 0))
          (make-filled-box-stencil
           (cons (cdr xext) (+ (cdr xext) thick))
           yext)
          empty-stencil)
      (if (and (not open-on-left) (> thick 0))
          (make-filled-box-stencil
           (cons (- (car xext) thick) (car xext))
           yext)
          empty-stencil)
      )))

#(define (music-boxer-stencil grob)
   (let* ((elts (ly:grob-object grob 'elements))
          (refp-X (ly:grob-common-refpoint-of-array grob elts X))
          (X-ext (ly:relative-group-extent elts refp-X X))
          (refp-Y (ly:grob-common-refpoint-of-array grob elts Y))
          (Y-ext (ly:relative-group-extent elts refp-Y Y))
          (padding (ly:grob-property grob 'padding 0.3))
          (thick (ly:grob-property grob 'thickness 0.1))
          (filled (ly:grob-property grob 'filled #f))
          (fill-color (ly:grob-property grob 'fill-color grey))
          (offset (ly:grob-relative-coordinate grob refp-X X))
          ; (left-bound  (ly:spanner-bound grob LEFT))
          ; (right-bound (ly:spanner-bound grob RIGHT))
          ; (break-dir-L (ly:item-break-dir left-bound))
          ; (break-dir-R (ly:item-break-dir right-bound))
          ; (open-on-left  (if (=  1 break-dir-L) #t #f))
          ; (open-on-right (if (= -1 break-dir-R) #t #f))
          (open-on-left
           (and (ly:spanner? grob)
                (= 1 (ly:item-break-dir (ly:spanner-bound grob LEFT)))))
          ; (open-on-left 
          ; (if (ly:spanner? grob)
          ; (if (=  1 (ly:item-break-dir (ly:spanner-bound grob LEFT)))
          ;    #t #f)
          ;  #f))
          (open-on-right
           (and (ly:spanner? grob)
                (= -1 (ly:item-break-dir (ly:spanner-bound grob RIGHT)))))
          ;(open-on-right
          ;(if (ly:spanner? grob)
          ;   (if (= -1 (ly:item-break-dir (ly:spanner-bound grob RIGHT)))
          ;      #t #f)
          ; #f))
          (stil (make-box thick padding filled fill-color
                  open-on-left open-on-right X-ext Y-ext))
          )
     (ly:stencil-translate-axis stil (- offset) X)
     )
   )