3.1.51 HorizontalBracket  HorizontalBracket objects are created
by: Horizontal_bracket_engraver.  Standard settings:  bracket-
flare (pair of numbers):  '(0.5 . 0.5)   connect-to-neighbor
(pair):   Pair of booleans, indicating whether this grob looks
as a continued break.  direction (direction):    padding
(dimension, in staff space):    side-axis (number):   If the
value is X (or equivalently 0), the object is placed
horizontally next to the other object. If the value is Y or 1,
it is placed vertically.  staff-padding (dimension, in staff
space):    stencil (stencil):   The symbol to print.  thickness
(number):   Line thickness, generally measured in line-
thickness.  Y-offset (number):    This object supports the
following interface(s): grob-interface, horizontal-bracket-
interface, line-interface, side-position-interface and spanner-
interface.  Internals Reference