The expression E cast as T converts the value of expression E to type
T. Whether T is a built-in type or a user-defined type, the
effect is exactly the same as using the constructor function T (E).
The expression E treat as T is designed for environments that perform static type
checking. Saxon doesn't do static type checking, so this expression has very little use, except to
document an assertion that the expression E is of a particular type. A run-time failure
will be reported if the value of E is not of type T; no attempt is made
to convert the value to this type.
