tuplet-bracket-interface
A bracket with a number in the middle, used for tuplets. When the bracket spans  a line break, the value of break-overshoot determines how far it extends beyond the staff. At a line break, the markups in the edge-text are printed at the edges.
User settable properties:
     
bracket-flare (pair of numbers)- A pair of numbers specifying how
much edges of brackets should slant outward.  Value 0.0 means straight
edges
 bracket-visibility (boolean or symbol)- This controls the
visibility of the tuplet bracket.  Setting it to false will prevent
printing of the bracket. Setting the property to 
'if-no-beam
will make it print only if there is no beam associated with this
tuplet bracket. 
 break-overshoot (pair of numbers)- How much does a broken spanner
stick out of its bounds? 
 connect-to-neighbor (pair)- Pair of booleans, indicating whether this
grob looks as a continued break. 
 control-points (list)- List of offsets (number-pairs) that form
control points for the tie/slur/bracket shape. For beziers, this
should list the control points of a 3rd order bezier curve. 
 direction (direction)- If side-position is 1 (#X), then this property determines if the object is placed #LEFT, #CENTER or #RIGHT with respect to the other object. Otherwise, it determines if the object is placed #UP #CENTER or #DOWN.  Numerical values may also be used. #UP=1, #DOWN=-1, #LEFT=-1, #RIGHT=1, CENTER=0 but also other numerical values are permitted. 
 edge-height (pair)- A pair of number specifying the heights of
the vertical edges '(left-height . right-height). 
 edge-text (pair)- A pair specifying the texts to be set at the
edges '(left-text . right-text). 
 gap (dimension, in staff space)- Size of a gap in a variable symbol. 
 padding (dimension, in staff space)- Add this much extra space between
objects that are next to each other. 
 positions (pair)- Pair of staff coordinates 
(left
. right), where both left and right are in the
staff-space unit of the current staff. 
 shorten-pair (pair of numbers)- The lengths to shorten a
text-spanner on both sides, for example a pedal bracket.  Positive values
shorten the text-spanner, while negative values lengthen it. 
 staff-padding (dimension, in staff space)- Maintain this much space between reference points
and the staff.  Its effect is to align objects of differing
sizes (like the dynamic p and f) on their baselines. 
 thickness (number)- Line thickness, generally measured in
line-thickness. 
 
Internal properties:
     
note-columns (pair)- list of NoteColumn grobs. 
 tuplet-number (layout object)- the number for a bracket. 
 tuplets (unknown)- list of smaller tuplet brackets
 
   This grob interface is used in the following graphical objects: LigatureBracket and TupletBracket