Re: [Lilypond-auto] Issue 3512 in lilypond: [DOC]Eample in NR 5.5.5 Unpure-pure containers is misleading

[lilypond]

Comment #1 on issue 3512 by address@hidden: [DOC]Eample in NR 5.5.5  
Unpure-pure containers is misleading
http://code.google.com/p/lilypond/issues/detail?id=3512

The concept that I needed to figure out was this :

The layout engine looks up some properties tentatively, when evaluating  
layout options.  Y-extent is one example of such a property.

If these properties are set to callback functions, the layout engine calls  
the function with arguments (Grob, start-column, end-column) to request the  
value of the property if the Grob ends up on a line between the given  
columns, and with a single argument (Grob) to request the final value.

If we override any of these properties with a function, that function must  
accept both forms of argument list.  The unpure-pure-container is a  
mechanism to meet this requirement.

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