Issue 4024: Clarify break-align symbols and space-alist args in IR.

Things take so long to understand when they're not documented!

Issue 4024 on the tracker:
http://code.google.com/p/lilypond/issues/detail?id=4024

[Mark Polesky, 2014-07-27 22:42:12]

Since no one reviewed this patch, I'm setting its status back to review.  I'll
push it next countdown if I all remains quiet.

Thanks,
Mark

[Keith, 2014-07-29 05:35:40]

Looks good, except for details I couldn't confirm, noted below.  If you
confirmed them, looks good.

https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.scm
File scm/define-grob-properties.scm (right):

https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
scm/define-grob-properties.scm:902: @code{right-edge}; otherwise it is fixed.
Looking at the code, 'right-edge' would seem to be among the "otherwise it is
fixed" cases.  If you found differently by experiment, then experiment rules.

https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
scm/define-grob-properties.scm:905: Put at least this much space between the
left side of both grobs,
"left sides"

https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
scm/define-grob-properties.scm:911: Only use with @code{first-note},
@code{next-note}, and @code{right-edge}.
"Only effective"  communicates more information.  "Only use" makes me wonder "or
else what?"

https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
scm/define-grob-properties.scm:917: the note (or edge), without allowing them to
collide.
Again, I don't see 'right-edge' in the cases that read minimum-fixed-space, nor
semi-fixed-space.

[Mark Polesky, 2014-07-29 20:12:47]

On 2014/07/29 05:35:40, Keith wrote:
>
https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
> scm/define-grob-properties.scm:902: @code{right-edge}; otherwise it is fixed.
> Looking at the code, 'right-edge' would seem to be among the "otherwise it is
> fixed" cases.  If you found differently by experiment, then experiment rules.

You are correct.  Done.

>
https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
> scm/define-grob-properties.scm:905: Put at least this much space between the
> left side of both grobs,
> "left sides"

Done.

>
https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
> scm/define-grob-properties.scm:911: Only use with @code{first-note},
> @code{next-note}, and @code{right-edge}.
> "Only effective"  communicates more information.  "Only use" makes me wonder
"or
> else what?"

I decided to use "only compatible with", since technically some combinations are
"effective" even though they weren't intended to be (see below).

>
https://codereview.appspot.com/114160044/diff/1/scm/define-grob-properties.sc...
> scm/define-grob-properties.scm:917: the note (or edge), without allowing them
to
> collide.
> Again, I don't see 'right-edge' in the cases that read minimum-fixed-space,
nor
> semi-fixed-space.

Ah, I see.  Here's the confusion: when paired with right-edge, *all* 5 of the
spacing-styles actually do something (to be clear, they all do the same thing:
they behave like extra-space, with space that doesn't stretch).  Anyway, am I
correct in concluding that right-edge is only intended to work with extra-space?
 I've edited the patch according to that understanding; please review and
comment again if you would.

For some reason Rietveld doesn't show the deltas from the previous patch set
(maybe because I rebased?), so you'll just have to read it again.  The only
changes are in lines 891-926 here:
http://codereview.appspot.com/114160044/diff/60001/scm/define-grob-properties...

Thanks for looking this over,
Mark

[Keith, 2014-07-30 04:59:36]

On Tue, 29 Jul 2014 13:12:47 -0700, <markpolesky@gmail.com> wrote:

> Ah, I see.  Here's the confusion: when paired with right-edge, *all* 5
> of the spacing-styles actually do something (to be clear, they all do
> the same thing: they behave like extra-space, with space that doesn't
> stretch).  Anyway, am I correct in concluding that right-edge is only
> intended to work with extra-space?  I've edited the patch according to
> that understanding; please review and comment again if you would.
>
That looks like what the code in 'break-alignment-interface.cc' would do.

When I worked on the spacing code, the behavior of these space-alist settings
was maddening, and I had always planned to simplify them.  I like the idea of
documenting them now, because that summarizes what we would have to think about
in a convert-ly rule if we do simpify them.

> For some reason Rietveld doesn't show the deltas from the previous patch
> set (maybe because I rebased?), so you'll just have to read it again.

We can still see that delta if we ask for it in the pull-down menus :
https://codereview.appspot.com/114160044/diff2/1:60001/scm/define-grob-proper...