ProcessLeavingTail: called to do the real work of this instruction. This method
must be implemented in each subclass. The results of the instruction are written
to the current Receiver, which can be obtained via the Controller.
output - the destination for the result
context - The dynamic context of the transformation, giving access to the current node,
the current variables, etc.
null if the instruction has completed execution; or a TailCall indicating
a function call or template call that is delegated to the caller, to be made after the stack has
been unwound so as to save stack space.
XPathException - if a dynamic error occurs during the evaluation
of the instruction