#(define* (center-stencil-on-stencil stil-a stil-b #:optional axis)
   "Return a copy of stencil @var{stil-b} that has been moved 
   so it is centered on stencil @var{stil-a} on @var{axis}. When
   @var{axis} is omitted, the centering is done on both X and Y
   axes. @var{axis} can be X or Y, which are equivalent to 0 and 1."
   (if axis
       ;; center on one axis only
       (ly:stencil-translate-axis
        (ly:stencil-aligned-to stil-b axis CENTER)
        (interval-center (ly:stencil-extent stil-a axis))
        axis)
       ;; center on both X and Y axes
       (ly:stencil-translate
        (centered-stencil stil-b)
        (cons
         (interval-center (ly:stencil-extent stil-a X))
         (interval-center (ly:stencil-extent stil-a Y))))))

square =
#(make-connected-path-stencil
  '((0 0) (4 0) (4 4) (0 4) (0 0))
  0.4 1 1 #f #f)

blue-square =
#(stencil-with-color
  (make-filled-box-stencil '(0.4 . 2) '(0.4 . 2))
  blue)

\markup
\override #'(word-space . 3)
\line {
  \stencil #(ly:stencil-add square blue-square)
  \stencil #(ly:stencil-add square
              (center-stencil-on-stencil square blue-square X))
  \stencil #(ly:stencil-add square
              (center-stencil-on-stencil square blue-square Y))
  \stencil #(ly:stencil-add square
              (center-stencil-on-stencil square blue-square))
}