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## Re: Issue 5621: Improve rehearsal mark position at beginning of staff (i

 From: Carl Sorensen Subject: Re: Issue 5621: Improve rehearsal mark position at beginning of staff (issue 547340043 by address@hidden) Date: Thu, 19 Dec 2019 21:39:12 +0000 User-agent: Microsoft-MacOutlook/10.10.10.191111

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﻿On 12/19/19, 2:28 PM, "lilypond-devel on behalf of address@hidden"
wrote:

On 2019/12/17 18:49:41, dak wrote:
> On 2019/12/17 17:57:52, Dan Eble wrote:
> > I haven't looked very deeply into the potential use of an
unpure-pure
> container.
> >  It looks like unpure-pure containers are used for vertical
placement,

> They are for things that are dependent on line-breaking.  I should
have thought
> this might apply here.

I found a decent introduction to pure properties in the CG (10.13).

One last oddity is that purity, in LilyPond, is currently
limited exclusively to things that have to do with Y-extent
and positioning. There is no concept of ‘pure X’ as, by
design, X is always the independent variable (i.e. from
column X1 to column X2, what will be the Y height of a
given grob).

This seems to support my conclusion that no further work is required.

Although I am not an expert in pure/impure calculations, I think that you are
misunderstanding this entry in the CG.

Because different line breaking can move the rehearsal mark in the X direction,
the Y skyline of the  system depends on where the rehearsal mark is placed.
But the rehearsal mark will potentially be placed somewhere different depending
on the line breaking.  But the line breaking is dependent on the Y skyline.  So
we end up with circular logic.

The pure container allows an estimate of the Y skyline to be calculated,
without asking for the grob to be recalculated.

Of course, this could all be wrong, but it's my best understanding of how
pure/impure works.

Carl

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