@layer(optional)Identifies the layer to which a feature applies.
One or more values of datatype positiveInteger, separated by spaces.
@plist(optional)When the target attribute is present, plist identifies the active participants; that
is, those entities pointed "from", in a relationship with the specified target(s).
When
the target attribute is not present, it identifies participants in a mutual
relationship.
One or more values from data.URI, separated by spaces.
@staff(optional)Signifies the staff on which a notated event occurs or to which a control event
applies. Mandatory when applicable.
One or more values of datatype positiveInteger, separated by spaces.
@tstamp(optional)Encodes the onset time in terms of musical time, i.e., beats[.fractional beat part],
as expressed in the written time signature.
Value conforms to data.BEAT.