They do not use the parent property, but instead use the 'group' property to form a collection of objects over which the aggregate is taken. The following properties are also available for collectors
group specifies the grouping rule for creating the collection. Groups may be specified using property condition value, where property is one of class, size, parent, id, rank, or any registered property of the object.property value of collectors must be in the form aggregator(property) where the aggregator is one of min, max, count, avg, std, mean, var, mbe, kur. If a property.part, where part is one of real, imag, mag, ang, or arg. Angles are in degrees, and arg is in radians.