dur(optional)When a duration cannot be represented as a single power-of-two value, multiple
space-separated values that add up to the total duration may be used.
One or more values from data.DURATION, separated by spaces.
layer(optional)Identifies the layer to which a feature applies.
One or more values of datatype positiveInteger, 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.