When only 'vtrack' argument is used in the call, the functions return the
corresponding attribute of the virtual track. Otherwise a new attribute
value is set.
Note: since inter-dependency exists between certain attributes, the
correctness of the attributes as a whole can only be verified when the
virtual track is used in a track expression.
For more information about the valid attribute values please refer to the
documentation of 'emr_vtrack.create'.