(y1 (- half-thick))
(y2 (+ top-fret-thick half-thick))
(x-extent (cons (- x1) x2))
- (y-extent (cons 0 y2)))
+ (y-extent (cons sth top-fret-thick)))
(if (eq? orientation 'normal)
(ly:make-stencil (list 'round-filled-box x1 x2 y1 y2 sth)
x-extent y-extent)