box widths

[Werner LEMBERG]

I have a fundamental problem with the width of lilypond output, and
which should be fixed before releasing 2.4.  Currently, I use

  (interval-start (ly:stencil-extent page X))

but this isn't the right value.  Reason:  Contrary to TeX boxes,
lilypond's horizontal reference point is not always the left edge of
the output box:

                     ^
                     |
                +----+----------+
                |    |          |
                |    |          |
            ----+----+----------+--->
                |    |          |
                +----+----------+
                     |
                     |

We have `height' and `depth' as with TeX boxes, and we have `width'
and `extension' (to coin this term), where `width' is the amount right
of the y-axis, and `extension' the left amount (and let's assume that
`extension' is positive).

The above scheme code gives `width + extension', but I need `width'
only.  How can I get that?


    Werner