and/or crashes.
-@node Defining context defaults
-@subsection Defining context defaults
+@node Changing context default settings
+@subsection Changing context default settings
The adjustments of the previous chapters can also be entered separate
from the music, in the @code{\paper} block,
@end example
@noindent
-takes the existing definition @context{Staff}, and adds
-changes some settings,
+takes the existing definition @context{Staff} from the identifier
+@code{StaffContext}. This works analogously other contexts, so the
+existing definition of @code{Voice} is in
+@code{\VoiceContext}.
+The statements
@example
\set fontSize = #-2
\override Stem #'thickness
\remove "Time_signature_engraver"
@end example
+@noindent
+affect all staves in the score.
+
+The @code{\set} keyword is optional within the @code{\paper} block, so
+
+@example
+ fontSize = #-2
+@end example
+
+@noindent
+will also work.
+
+
+
+
+@node Defining new contexts
+@subsection Defining new contexts
+
+Specific contexts, like @context{Staff} and @code{Voice} are made of
+simple building blocks, and it is possible to compose engraver
+plug-ins in different combinations, thereby creating new types of
+contexts.
+
+The next example shows how to build a different type of
+@context{Voice} context from scratch. It will be similar to
+@code{Voice}, but print centered slash noteheads only. It can be used
+to indicate improvisation in Jazz pieces,
+
+@lilypond[raggedright]
+ \paper { \context {
+ \name ImproVoice
+ \type "Engraver_group_engraver"
+ \consists "Note_heads_engraver"
+ \consists "Text_script_engraver"
+ \consists Pitch_squash_engraver
+ squashedPosition = #0
+ \override NoteHead #'style = #'slash
+ \override Stem #'transparent = ##t
+ \alias Voice
+ }
+ \context { \StaffContext
+ \accepts "ImproVoice"
+ }
+ \score { \notes \relative c'' {
+ a4 d8 bes8 \new ImproVoice { c4^"ad lib" 4
+ c4 c^"undress" c c_"while playing :)" }
+ a1
+ }
+@end lilypond
+
+
+These settings are again done within a @code{\context} block inside a
+@code{\paper} block,
+
+@example
+ \paper {
+ \context {
+ @dots{}
+ }
+ }
+@end example
+
+In the following discussion, the example input shown should go on the
+@dots{} of the previous fragment.
+
+First, name the context gets a name. Instead of @context{Voice} it
+will be called @context{ImproVoice},
+@verbatim
+ \name ImproVoice
+@end verbatim
-Context properties can be set as defaults, within the
-@code{\paper} block. For example,
+Since it is similar to the @context{Voice}, we want commands that work
+on (existing) @context{Voice}s to remain working. This is achieved by
+giving the new context an alias @context{Voice},
+
+@verbatim
+ \alias Voice
+@end verbatim
+
+The context will print notes, and instructive texts
+
+@verbatim
+ \consists Note_heads_engraver
+ \consists Text_script_engraver
+@end verbatim
+
+but only on the center line,
+
+@verbatim
+ \consists Pitch_squash_engraver
+ squashedPosition = #0
+@end verbatim
+
+The @internalsref{Pitch_squash_engraver} modifies note heads (created
+by @internalsref{Note_heads_engraver}) and sets their vertical
+position to the value of @code{squashedPosition}, in this case
+@code{0}, the center line.
+
+The notes look like a slash, without a stem,
+
+@verbatim
+ \override NoteHead #'style = #'slash
+ \override Stem #'transparent = ##t
+@end verbatim
+
+
+All these plug-ins have to cooperate, and this is achieved with a
+special plug-in, which must be marked with the keyword @code{\type}.
+This should always be @internalsref{Engraver_group_engraver},
+
+@example
+ \type "Engraver_group_engraver"
+@end example
+
+Put together, we get
@verbatim
-\paper {
\context {
- \ScoreContext
- skipBars = ##t
+ \name ImproVoice
+ \type "Engraver_group_engraver"
+ \consists "Note_heads_engraver"
+ \consists "Text_script_engraver"
+ \consists Pitch_squash_engraver
+ squashedPosition = #0
+ \override NoteHead #'style = #'slash
+ \override Stem #'transparent = ##t
+ \alias Voice
}
-}
@end verbatim
-@noindent
-will set skipBars default
+Contexts form hierarchies. We want to hang the @consists{ImproVoice}
+under @consists{Staff}, just like normal @code{Voice}s. Therefore, we
+modify the @code{Staff} definition with the @code{\accepts}
+command,@footnote{The opposite of @code{\accepts} is @code{\denies},
+which is sometimes when reusing existing context definitions. }
+
+
+
+@verbatim
+ \context {
+ \StaffContext
+ \accepts ImproVoice
+ }
+@end verbatim
+
+
+@example
+
+
@node Which properties to change
@subsection Which properties to change
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