This is the abstract base class for all state elements. This class defines the following features for all of its derived classes:

<B> Type identifiers: </B> Each class of element has a unique (static)
SoType identifier. The correct type id is also stored in each
instance for easy access.

<B> Copying: </B> Elements are copied into the list of elements used
in an SoCache. Performing any operation other than matches()
on a copied element is not guaranteed to work (and will not
work for things like Coordinates, if the coordinate they were
pointing to has been deleted).

<B> Stack indices: </B> Since a subclass of an element class needs to
be in the same state stack as the class from which it is
derived, stack indices are inherited.

<B> Capturing: </B> Each time an element's value is accessed, that
element is added to the elements-used list of all currently
open caches.

Reference