# The OpenLilyLib Edition Engraver


## What is it?  
In a nutshell, the Edition Engraver provides a convenient way of storing tweaks,
overrides and other objects that can later be applied to some musical content.

## Why use it?
To keep the "musical source" of a project free from tweaks, temporary overrides,
and tagged material that needs to be filtered later. This serves to generate
code that is clean, reusable and has clarity of purpose, meaning it's fast to
read and understand.

## How is it used?  

In summary, by following these four logical steps:  

  1. Load the Edition Engraver into the project.
  2. Create an edition (a container to store the edits)
  3. Fill the edition with content.
  4. Consist the contents of the edition to the musical contexts to which they apply.

Each step explained:  

### 1 Loading the Edition Engraver
Assuming OpenLilyLib is already installed on your working environment, include
its core functionality:

    \include "oll-core/package.ily"  

Then, load the Edition Engraver itself:  

    \loadPackage edition-engraver  

### 2 Creating an edition
Just use the `\addEdition` command. Like this:  

    \addEdition edition-name  

### 3 Filling the edition with content
The most basic way to do this is by using the _\editionMod_ command. It is
used as follows:  


    \editionMod edition measure position context content  

Breaking it apart:  

*  `edition` specifies in what edition the content is stored.
*  `measure` specifies in what measure of the music the content is to be placed.
*  `position` specifies where where exactly in that measure the content is to be placed.
*  `context` specifies in what context the content belongs.
*  `content` specifies, finally, what should be placed there.

So, this means that  

    \editionMod my-edition 5 0/4 Score \break  

will store in `my-edition` that a `\break` needs to be placed in the `Score`
context, in measure `5` , specifically at `0/4`, which is its first beat.  

#### 3.1 About the position value
This is the amount of musical time that is counted from the start of the given
measure. A few useful examples:

* `0/4` will not add anything, so it references the first beat of
  the measure.
* `3/8` will count three 8th notes / quavers from the start of the
  measure. In 4/4 time this would reference the second half of the second
  beat.
* `1/24` will count one 16th note / semiquaver of a 16th note triplet. If
  the measure starts with 16th note triplets, this will point to the second
  note of the measure. The fraction is expressed like this because there
  are 24  "tripleted 16th notes" in a whole note.

#### 3.2 About referencing contexts
Precise control can be achieved by giving ID's to contexts. This is done with
the `\editionID` command:  

    \new Staff \with { \editionID my-staff } {  
     \new Voice { c4 d e f }  
    }  

This ID can be used like this:  

    \editionMod test 1 2/4 my-staff.Staff \accidentalStyle dodecaphonic  
    \editionMod test 1 3/4 my-staff.Voice.A \override NoteHead.color = #red  

Notice that even though the ID `my-staff` points to a specific `Staff`,
`\editionMod` still needs to know specifically where you need to inject the
content. So, `my-staff.Staff` puts it in the `Staff` context, while
`my-staff.Voice.A` puts it in the first `Voice` inside the `Staff`.

Voices are listed in the order they are created, starting with the symbol `A`,
and each `Staff` keeps a separate count. The Edition Engraver produces a
".edition.log" file listing all the contexts it finds along with their names.
This is useful to work with music that requires spontaneous creation of Voices,
such as piano music, where naming them provides a straightforward way of finding
how to reference them.

Say you have an "example.ly" file with the following
music:  

    \new Staff \with { \editionID my-staff } {  
     \new Voice = "main-voice" \relative c'' {  
     c4 d e f  
     << { \voiceOne e d c2 }  
     \new Voice = "spontaneous-voice" { \voiceTwo g2 e } >>  
     }  
    }  

This will produce an "example.edition.log" file that, among other things, will
have the following:  

    (my-staff Voice A) "main-voice"  
    (my-staff Voice B) "spontaneous-voice"  

While the `Score` context can't be instantiated, an ID can be given to a
`\score` block in it 's `\layout` block, like this:  

    \score {  
     ...  music goes here ...  
     \layout {  
     \context {  
     \Score  
     \editionID my-score  
     }  
     }  
    }  
>
This very useful when you need specific edits for parts that you want to keep
out of the full score. While references can grow long pretty quickly,
fortunately they can be stored in variables:

    referenceOne = my-score.my-staff.Voice.A  
    referenceTwo = my-other-score.my-staff.Voice.B  
>
And then used like this:  

    \editionMod test 1 3/4 \referenceOne ->

##### 3.2.1 A warning
Keep bottom level content to bottom level contexts. See this example:  

    \version "2.19.82"  

    \include "oll-core/package.ily"  
    \loadPackage edition-engraver  

    \addEdition example  
    \editionMod example 1 0/4 good-staff.Voice.A \once \override NoteHead.color = #red  
    \editionMod example 1 0/4 bad-staff.Voice.A \once \override NoteHead.color = #red  
    \consistToContexts #edition-engraver Staff.Voice  

    \score {  
     \new StaffGroup <<  
     \new Staff \with { \editionID good-staff } {  
     \new Voice { \clef C c' d' e' f' }  
     }  
     \new Staff \with { \editionID bad-staff } {  
     \clef C \new Voice { c' d' e' f' }  
     }  
     >>  
    }  

Running this shows that `good-staff` has a red first note, while `bad-staff`
doesn't. This is because on the second `Staff` the `\clef` command is tacitly in
a `Voice` that has no length. This kind of tacit Voice creation produces strange
results. This is easily avoided by moving the `\clef` command inside the Voice
that has actual music, like `good-staff` shows.  

### 4 Consisting to contexts  
This is done with the `\consistToContexts` command, like this:  

    \consistToContexts #edition-engraver contexts.separated.by.dots  

The contexts absent from the dot-separated list will not be modified. So, for
instance:

    \editionMod test 1 0/4 my-score.Score \tempo "Adagio."  
    \editionMod test 1 0/4 my-other-score.my-staff.Staff \tempo "Adagio."  
    \consistToContexts #edition-engraver Staff.Voice  

will create a `TempoChangeEvent` in `my-other-score`, since the instruction
is to create it in the `Staff` context, but leave `my-score` untouched.  

Hopefully this will be enough to get anyone started. There is more functionality
than this. For a start, examine the `usage-examples` folder in the Openlilylib
Edition Engraver repository.