same pages in a4 and letter

[Graham Percival]

I'm preparing my old scores for online publication, but I'm
extremely fussy about engraving.  I want users on both sides of
the Atlantic to be able to print my music easily, but this only
works if the music looks in the same in A4 and Letter paper.
(having page turns in random places is *not* acceptable to me :)

The attached file achieves this:
- creates A4 (default) or letter (from a command-line option)
// shamelessly stolen from Nicholas Sceaux's scores

- automatically adjusts the margins such that there's at least a
  1cm margin.  A4 has smaller horizontal margins, letter has
  smaller vertical margins.

- produces a usable area (for headers, music, text) of 190mm by
  259mm.

Let me know if you can think of any improvements; I'll add it to
LSR when I'm reasonably confident that this is the best way to go
about this.

Cheers,
- Graham

spacing-a4-letter.ly
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