\version "2.19.50"

\version "2.19.49"

#(define (lists-map function ls)
   "Apply @var{function} to @var{ls} and all of it sublists. 
First it recurses over the children, then the function is applied to 
@var{ls}."
   (if (list? ls)
       (set! ls (map (lambda (y) (lists-map function y)) ls))
       ls)
   (function ls))

#(define (change-width x y)
   "Adds values @var{x} and @var{y} to certain elements of a list, which starts 
with @code{'draw-line}."
   (lambda (e)
     (if (and (list? e) (member 'draw-line e))
         `(,@(take e 3)
            ,((if (negative? (fourth e)) + -) (fourth e) y)
            ,(if (not (zero? (fifth e))) (+ x (fifth e)) (fifth e))
            ,((if (negative? (last e)) + -) (last e) y))
         e)))

#(define* (resize-square x #:optional (y #t))
   "Applies @code{change-width} to the stencil-expression of a grob, which is 
supposed to be SystemStartSquare. 

@var{x} will resize the length of the horizontal wings. 

The optional @var{y} affects the general size in Y-direction, it's default is 
@code{#t}: 
- SystemStartSquare is off by half staff-line-thickness compared to the staves, 
this default may or may not be intended. The default for @var{y} corrects it. 
- If set to a number the size is freely customizable. 
- If set to false, the size in Y-direction is not touched. 

Using this procedure causes the need to reset the padding of the 
SystemStartSquare. 
"
   (lambda (grob)
     (let* ((default-stil (ly:system-start-delimiter::print grob))
            (default-stil (if (ly:stencil? default-stil) default-stil empty-stencil))
            (staff-line-thick (ly:staff-symbol-line-thickness grob))
            (y-corr
             (cond ((number? y) (- y))
               ((eq? #t y) (/ staff-line-thick 2))
               (else 0))))
       ;; n.b.
       ;; the final stencil has the same x- and y-extent as the original
       (ly:make-stencil
        (lists-map (change-width x y-corr) (ly:stencil-expr default-stil))
        (ly:stencil-extent default-stil X)
        (ly:stencil-extent default-stil Y)))))

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% EXAMPLE
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\new StaffGroup
\with {
  \override SystemStartBracket.padding = 0.5
  systemStartDelimiter = #'SystemStartBracket
}
<<
  \new Staff { c'4 }
  \new StaffGroup
  \with {
    %% next two commands for better visibility only
    %\override SystemStartSquare.color = #cyan
    %\override SystemStartSquare.layer = 2000
    \override SystemStartSquare.stencil = #(resize-square 0.8)
    \override SystemStartSquare.padding = -0.2
    systemStartDelimiter = #'SystemStartSquare
  }
  <<
    \new StaffGroup
    \with {
      %% next command for better visibility only
      %\override SystemStartSquare.color = #red
      \override SystemStartSquare.stencil = #(resize-square 1.6)
      \override SystemStartSquare.padding = -0.2
      systemStartDelimiter = #'SystemStartSquare
    }
    <<
      \new Staff { c'4 }
      \new Staff  \with {   \RemoveAllEmptyStaves } {  }
    >>

    \new Staff { c'4 }
  >>
  \new Staff { c'4 }
>>