Comment #1 on issue 3512 by [email protected]: [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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