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<h4 class="unnumberedsubsec">item-interface</h4>
<p>Grobs can be distinguished in their role in the horizontal spacing. Many grobs define constraints on the spacing by their sizes, for example, note heads, clefs, stems, and all other symbols with a fixed shape. These grobs form a subtype called <code>Item</code>.
<p>Some items need special treatment for line breaking. For example, a clef is normally only printed at the start of a line (i.e., after a line break). To model this, ‘breakable’ items (clef, key signature, bar lines, etc.) are copied twice. Then we have three versions of each breakable item: one version if there is no line break, one version that is printed before the line break (at the end of a system), and one version that is printed after the line break.
<p>Whether these versions are visible and take up space is determined by the outcome of the <code>break-visibility</code> grob property, which is a function taking a direction (<code>-1</code>, <code>0</code> or <code>1</code>) as an argument. It returns a cons of booleans, signifying whether this grob should be transparent and have no extent.
<p>The following variables for <code>break-visibility</code> are predefined:
<pre class="example"> grob will show: before no after
break break break
all-invisible no no no
begin-of-line-visible no no yes
end-of-line-visible yes no no
all-visible yes yes yes
begin-of-line-invisible yes yes no
end-of-line-invisible no yes yes
center-invisible yes no yes
</pre>
<h5 class="unnumberedsubsubsec">User settable properties:</h5>
<blockquote>
<dl>
<dt><code>break-visibility</code> (vector)<dd>A vector of 3 booleans,
<code>#(</code><var>end-of-line</var> <var>unbroken</var> <var>begin-of-line</var><code>)</code>.
<code>#t</code> means visible, <code>#f</code> means killed.
<br><dt><code>extra-spacing-height</code> (pair of numbers)<dd>In the horizontal spacing
problem, we increase the height of each item by this amount (by adding
the ‘car’ to the bottom of the item and adding the ‘cdr’ to the top
of the item. In order to make a grob infinitely high (to prevent the
horizontal spacing problem from placing any other grobs above or below
this grob), set this to <code>(-inf.0 . +inf.0)</code>.
<br><dt><code>extra-spacing-width</code> (pair of numbers)<dd>In the horizontal spacing
problem, we pad each item by this amount (by adding the ‘car’ on the
left side of the item and adding the ‘cdr’ on the right side of the
item). In order to make a grob take up no horizontal space at all,
set this to <code>(+inf.0 . -inf.0)</code>.
<br><dt><code>non-musical</code> (boolean)<dd>True if the grob belongs to a
<code>NonMusicalPaperColumn</code>.
</dl>
</blockquote>
<p>This grob interface is used in the following graphical object(s): <a href="Accidental.html#Accidental">Accidental</a>, <a href="AccidentalCautionary.html#AccidentalCautionary">AccidentalCautionary</a>, <a href="AccidentalPlacement.html#AccidentalPlacement">AccidentalPlacement</a>, <a href="AccidentalSuggestion.html#AccidentalSuggestion">AccidentalSuggestion</a>, <a href="Ambitus.html#Ambitus">Ambitus</a>, <a href="AmbitusAccidental.html#AmbitusAccidental">AmbitusAccidental</a>, <a href="AmbitusLine.html#AmbitusLine">AmbitusLine</a>, <a href="AmbitusNoteHead.html#AmbitusNoteHead">AmbitusNoteHead</a>, <a href="Arpeggio.html#Arpeggio">Arpeggio</a>, <a href="BalloonTextItem.html#BalloonTextItem">BalloonTextItem</a>, <a href="BarLine.html#BarLine">BarLine</a>, <a href="BarNumber.html#BarNumber">BarNumber</a>, <a href="BassFigure.html#BassFigure">BassFigure</a>, <a href="BassFigureBracket.html#BassFigureBracket">BassFigureBracket</a>, <a href="BreakAlignGroup.html#BreakAlignGroup">BreakAlignGroup</a>, <a href="BreakAlignment.html#BreakAlignment">BreakAlignment</a>, <a href="BreathingSign.html#BreathingSign">BreathingSign</a>, <a href="ChordName.html#ChordName">ChordName</a>, <a href="Clef.html#Clef">Clef</a>, <a href="ClusterSpannerBeacon.html#ClusterSpannerBeacon">ClusterSpannerBeacon</a>, <a href="CombineTextScript.html#CombineTextScript">CombineTextScript</a>, <a href="Custos.html#Custos">Custos</a>, <a href="DotColumn.html#DotColumn">DotColumn</a>, <a href="Dots.html#Dots">Dots</a>, <a href="DoublePercentRepeat.html#DoublePercentRepeat">DoublePercentRepeat</a>, <a href="DoublePercentRepeatCounter.html#DoublePercentRepeatCounter">DoublePercentRepeatCounter</a>, <a href="DynamicText.html#DynamicText">DynamicText</a>, <a href="Fingering.html#Fingering">Fingering</a>, <a href="FretBoard.html#FretBoard">FretBoard</a>, <a href="GridLine.html#GridLine">GridLine</a>, <a href="GridPoint.html#GridPoint">GridPoint</a>, <a href="HarmonicParenthesesItem.html#HarmonicParenthesesItem">HarmonicParenthesesItem</a>, <a href="InstrumentSwitch.html#InstrumentSwitch">InstrumentSwitch</a>, <a href="KeyCancellation.html#KeyCancellation">KeyCancellation</a>, <a href="KeySignature.html#KeySignature">KeySignature</a>, <a href="LaissezVibrerTie.html#LaissezVibrerTie">LaissezVibrerTie</a>, <a href="LaissezVibrerTieColumn.html#LaissezVibrerTieColumn">LaissezVibrerTieColumn</a>, <a href="LeftEdge.html#LeftEdge">LeftEdge</a>, <a href="LyricText.html#LyricText">LyricText</a>, <a href="MelodyItem.html#MelodyItem">MelodyItem</a>, <a href="MetronomeMark.html#MetronomeMark">MetronomeMark</a>, <a href="NonMusicalPaperColumn.html#NonMusicalPaperColumn">NonMusicalPaperColumn</a>, <a href="NoteCollision.html#NoteCollision">NoteCollision</a>, <a href="NoteColumn.html#NoteColumn">NoteColumn</a>, <a href="NoteHead.html#NoteHead">NoteHead</a>, <a href="NoteName.html#NoteName">NoteName</a>, <a href="NoteSpacing.html#NoteSpacing">NoteSpacing</a>, <a href="OctavateEight.html#OctavateEight">OctavateEight</a>, <a href="PaperColumn.html#PaperColumn">PaperColumn</a>, <a href="ParenthesesItem.html#ParenthesesItem">ParenthesesItem</a>, <a href="RehearsalMark.html#RehearsalMark">RehearsalMark</a>, <a href="RepeatSlash.html#RepeatSlash">RepeatSlash</a>, <a href="RepeatTie.html#RepeatTie">RepeatTie</a>, <a href="RepeatTieColumn.html#RepeatTieColumn">RepeatTieColumn</a>, <a href="Rest.html#Rest">Rest</a>, <a href="RestCollision.html#RestCollision">RestCollision</a>, <a href="Script.html#Script">Script</a>, <a href="ScriptColumn.html#ScriptColumn">ScriptColumn</a>, <a href="ScriptRow.html#ScriptRow">ScriptRow</a>, <a href="SeparationItem.html#SeparationItem">SeparationItem</a>, <a href="SostenutoPedal.html#SostenutoPedal">SostenutoPedal</a>, <a href="SpanBar.html#SpanBar">SpanBar</a>, <a href="StaffSpacing.html#StaffSpacing">StaffSpacing</a>, <a href="StanzaNumber.html#StanzaNumber">StanzaNumber</a>, <a href="Stem.html#Stem">Stem</a>, <a href="StemTremolo.html#StemTremolo">StemTremolo</a>, <a href="StringNumber.html#StringNumber">StringNumber</a>, <a href="StrokeFinger.html#StrokeFinger">StrokeFinger</a>, <a href="SustainPedal.html#SustainPedal">SustainPedal</a>, <a href="TabNoteHead.html#TabNoteHead">TabNoteHead</a>, <a href="TextScript.html#TextScript">TextScript</a>, <a href="TimeSignature.html#TimeSignature">TimeSignature</a>, <a href="TrillPitchAccidental.html#TrillPitchAccidental">TrillPitchAccidental</a>, <a href="TrillPitchGroup.html#TrillPitchGroup">TrillPitchGroup</a>, <a href="TrillPitchHead.html#TrillPitchHead">TrillPitchHead</a> and <a href="UnaCordaPedal.html#UnaCordaPedal">UnaCordaPedal</a>.
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