This class represents the tuple stream delivered as the output of a where clause in a
FLWOR expression: that is, it supplies all the tuples in its input stream that satisfy
a specified predicate. It does not change the values of any variables in the tuple stream.
Notify the availability of the next tuple. Before calling this method,
the supplier of the tuples must set all the variables corresponding
to the supplied tuple in the local stack frame associated with the context object