Gets or sets the value associated with the specified key. In C#
this property is the indexer for the class.
The Item property is used to access Elements within the tree.
The tree is searched for a matching entry and used to create an
Element of type T.
Note that no factories are used here so the element will always
be type T - not a subclass.
Trees cannot hold null values in a meaningful way. So if you
provide a null value, this will result in an exception being
If the specified key is not found, a get operation throws a
KeyNotFoundException, and a set operation creates a new element
with the specified key.