Returns the type which generated the associated value. The type is important because it
allows easy access to the xml element inheritance tree, allowing the user to test whether it
is a valid data entry.
This returns the realized value of an element which was associated with this type. We
recommend that this value be realized prior to the first request for the value (use this
object to cache the result). If you do chose to implement this method lazily, consider
caching the result as it may be called more than once, expecting the same result both times.