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[Source files: command.hh, scommands.cc]
BREAKING, PREBREAK POSTBREAK, etc.
So what's the deal with PREBREAK and POSTBREAK and all this
stuff?
Let's take text as an example. In German some compound
words change their spelling if they are broken: "backen" becomes
"bak-ken". TeX has a mechanism to deal with this, you would define
the spelling of "backen" in TeX in this way
\discretionary{bak-}{ken}{backen}
These 3 arguments are called "prebreak", "postbreak" and "nobreak"
text.
The same problem exists when typesetting music. If a line of music is
broken, the next line usually gets a clef. So in TeX terms, the clef
is a postbreak. The same thing happens with meter signs: Normally the
meter follows the bar. If a line is broken at that bar, the bar along
with the meter stays on the "last" line, but the next line also gets a
meter sign after the clef. Using the previous notation,
\discretionary{bar meter}{clef meter}{ bar meter }
In Lilypond, we have the same concepts (and the same
terminology). Each (nonrhythmic) symbol is typeset using a Command
(code: TYPESET). At a breakpoint, TYPESET commands can be grouped
using separators (in lower case):
BREAK_PRE, typeset(bar), typeset(meter),
BREAK_MID, typeset(bar), typeset(meter),
BREAK_POST, typeset(clef), typeset(meter), BREAK_END
The BREAK command sequence is terminated with BREAK_END, so other
commands (like INTERPRET) may follow this sequence.
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