### 5.1.6 Defining new contexts

Specific contexts, like `Staff` and `Voice`, are made from simple building blocks. It is possible to create new types of contexts with different combinations of engraver plug-ins.

The next example shows how to build a different type of `Voice` context from scratch. It will be similar to `Voice`, but only prints centered slash note heads. It can be used to indicate improvisation in jazz pieces,

These settings are defined within a `\context` block inside a `\layout` block,

>     \layout {
>       \context {
>         …
>       }
>     }

In the following discussion, the example input shown should go in place of the … in the previous fragment.

First it is necessary to define a name for the new context:

>     \name ImproVoice

Since it is similar to the `Voice` context, we want commands that work in (existing) `Voice` contexts to continue working. This is achieved by giving the new context an alias of `Voice`,

>     \alias Voice

The context will print notes and instructive texts, so we need to add the engravers which provide this functionality, plus the engraver which groups notes, stems and rests which occur at the same musical moment into columns,

>     \consists "Note_heads_engraver"
>     \consists "Text_engraver"
>     \consists "Rhythmic_column_engraver"

The note heads should all be placed on the center line,

>     \consists "Pitch_squash_engraver"
>     squashedPosition = #0

The `Pitch_squash_engraver` modifies note heads (created by the `Note_heads_engraver`) and sets their vertical position to the value of `squashedPosition`, in this case `0`, the center line.

The notes look like a slash, and have no stem,

>     \override NoteHead.style = #'slash
>     \hide Stem

All these plug-ins have to communicate under the control of the context. The mechanisms with which contexts communicate are established by declaring the context `\type`. Within a `\layout` block, most contexts will be of type `Engraver_group`. Some special contexts and contexts in `\midi` blocks use other context types. Copying and modifying an existing context definition will also fill in the type. Since this example creates a definition from scratch, it needs to be specified explicitly.

>     \type "Engraver_group"

Put together, we get

>     \context {
>       \name ImproVoice
>       \type "Engraver_group"
>       \consists "Note_heads_engraver"
>       \consists "Text_engraver"
>       \consists "Rhythmic_column_engraver"
>       \consists "Pitch_squash_engraver"
>       squashedPosition = #0
>       \override NoteHead.style = #'slash
>       \hide Stem
>       \alias Voice
>     }

Contexts form hierarchies. We want to place the `ImproVoice` context within the `Staff` context, just like normal `Voice` contexts. Therefore, we modify the `Staff` definition with the `\accepts` command,

>     \context {
>       \Staff
>       \accepts ImproVoice
>     }

The opposite of `\accepts` is `\denies`, which is sometimes needed when reusing existing context definitions.

Putting both into a `\layout` block, like

>     \layout {
>       \context {
>         \name ImproVoice
>         …
>       }
>       \context {
>         \Staff
>         \accepts "ImproVoice"
>       }
>     }

Then the output at the start of this subsection can be entered as

>     \relative c'' {
>       a4 d8 bes8
>       \new ImproVoice {
>         c4^"ad lib" c
>         c4 c^"undress"
>         c c_"while playing :)"
>       }
>       a1
>     }

To complete this example, changes affecting the context hierarchy should be repeated in a `\midi` block so that Midi output depends on the same context relations.

#### See also

Internals Reference: Engraver_group, Note_heads_engraver, Text_engraver, Rhythmic_column_engraver, Pitch_squash_engraver.

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#### LilyPond — Notation Reference v2.18.2 (稳定分支).