endid(optional)Indicates the final element in a sequence of events to which the feature
applies.
Value conforms to data.URI.
level(optional)Indicates the nesting level of staff grouping symbols.
Value is a positive integer.
startid(optional)Holds a reference to the first element in a sequence of events to which the feature
applies.
Value conforms to data.URI.
symbol(optional)Specifies the symbol used to group a set of staves.
Allowed values are:
"brace" (Curved symbol, , {.), "bracket" (Square symbol, , [, but with curved/angled top and bottom segments.), "bracketsq" (Square symbol, , [, with horizontal top and bottom segments.), "line" (Line symbol, , |, (wide) line without top and bottom curved/horizontal
segments.), "none" (Grouping symbol missing.)