where supporting classes also find it convenient to invoke this
method. However, these are more likely to wish to invoke the <I>SemError(n)</I>
method with an appropriately chosen error number, which will report a semantic
error message related to the position of the most recently <I>parsed</I> token.  In
order to obtain meaningful messages, a programmer may conveniently derive a
suitable subclass from the <I>ErrorStream</I> class; see section 3.4.</P>

<P>A call to the method <I>Successful()</I> will retur