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@ignore
    Translation of GIT committish: FILL-IN-HEAD-COMMITTISH

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@node input-format 

@unnumberedsec Input format

As discussed earlier, the ideal input format for a music engraving
system is the content: the music itself. This poses a formidable
problem: how can we define what music really @emph{is}? Our way out
of this problem, is to reverse it.  Instead of defining what music is,
our program serves as a definition: we write a program capable of
producing sheet music, and adjust the format to be as lean as
possible.  When the format can no longer be trimmed down, by
definition we are left with content itself.

The syntax is also the user-interface for LilyPond, hence it is easily
typable, e.g.,

@verbatim

  c'4 d'8

@end verbatim
Are a quarter note C1 and eighth note D1, as in this example:

@divClass{float-center}
@sourceimage{simple-notation,,,.png}
@divEnd

On a microscopic scale, such syntax is easy to use. On a larger scale,
syntax also  needs structure.  How else can  you enter  complex pieces
like symphonies and operas? The  structure is formed by the concept of
music expressions:  by combining small fragments of  music into larger
ones, more complex music can be expressed.  For example,

@multitable @columnfractions .5 .5
@item

c4

@tab

@sourceimage{simultaneous-0,,,.png}


@end multitable

Combine this simultaneously with two other notes by enclosing in <&lt
and >>.

@multitable @columnfractions .5 .5

@item

@verbatim

  <<c4 d4 e4>>

@end verbatim


@tab

@sourceimage{simultaneous-1,,,.png}



@end multitable

This expression is put in sequence by enclosing it in braces, i.e.,
@multitable @columnfractions .5 .5
@item

@verbatim

   { <<c4 d4 e4>> f4  }
@end verbatim


@tab

@sourceimage{simultaneous-2,,,.png}



@end multitable

The above is another expression, and therefore, it may be combined 
again with a simultaneous expression (in this case, a half note). 

@multitable @columnfractions .5 .5
@item

@verbatim

<< { <<c4 d4 e4>> f4 } g2 >> 

@end verbatim


@tab

@sourceimage{simultaneous-3,,,.png}




@end multitable

Such recursive structures can be specified neatly and formally in a
@emph{context-free grammar}.  The parsing code is also generated
from this grammar. In other words, the  syntax of LilyPond is clearly
and unambiguously defined.

User-interfaces and syntax are what people see and deal with
most. They are partly a matter of taste, and also subject of much
discussion.  Although discussions on taste do have their merit, they
are not very productive.  In the larger picture of LilyPond, the
importance of input syntax is small: inventing neat syntax is easy,
writing decent formatting code is much harder.  This is also
illustrated by the line-counts for the respective components: parsing
and representation take up less than 10% of the code.

@multitable @columnfractions .5 .5
@item
Parsing + representation
@tab
@item
total
@tab
@item
6000 lines C++
@tab
61500 lines C++
@end multitable

@ignore
  TODO :

  blurbs about lilypond today

  future?
@end ignore

@divClass{float-right}
Next: @ref{conclusion,wrapping it up}, the conclusion.
@divEnd