This class represents a number tree node.
A number tree is a way of representing a mapping from integers
to other Elements.
The standard PDF dictionary object does something similar, but
it works natively in terms of strings rather than numbers and the
PDF specification implies that it might not have been optimized for
large numbers of items.
As such the PDF specification uses this type of tree structure,
in areas where there may be large numbers of keys and values.
This class is always an indirect object because EntryKids
requires it to be so.
This is definitively detailed in:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 37,