leftmargin and page layout

[Pedro Kroger]

Hi,

I implemented leftmargin in the \bookpaper block. rightmargin as an
option is still missing, but one can set it's value indirectly by
setting leftmargin and linewidth:

\bookpaper {
    leftmargin = 3\cm
    linewidth = 14\cm
}

if paper in this example is a4 (21 cm) rightmargin will be 4cm.

It seems that that if the \paper block exists the linewidth command
above will not work well.

I have a few questions:

1. what is the difference between \bookpaper and \paper? I see that
some variables can be set on both. As I mentioned above, if I had
both \bookpaper and \paper the result could be weired.

2. lilypond uses mainly hsize and linewidth to determine the size of
the side margins. for instance, in paper.scm:

    (module-define! m 'linewidth (- w (* 20 mm)))

the 20mm is the hardcoded default for the sum of both margins. I
believe that to have true left and right margins defined by the user
linewidth had to be defined in relation to them,
like linewidth = hzise-(lmargin+rmargin)

I believe that in most cases the uses wants to use
(top|bottom|left|right)margins instead of line(width|height).

The thing is (to implement rigthmargin):

1. linewidth is used in many places in the whole code, I don't think is
   an option to change them. 

2. I guess set-paper-dimensions could define linewidth this way, but I
   couldn't figure out how.

3. I figured that this could be set in default-page-make-stencil like
   leftmargin, but I couldn't figure out it handle
   linewidth (if it does).

Sorry if this is long, verbose, and basic, but I'm still trying to
understand how things work.

Pedro