<p>

Each symbol appearing in a production is represented at runtime by an
object of type <tt>Symbol</tt> on the parse stack.  The labels refer to
the instance variable <tt>value</tt> in those objects.  In the
expression <tt>expr:e1 PLUS expr:e2</tt>, <tt>e1</tt> and <tt>e2</tt>
refer to objects of type Integer.  These objects are in the value fields
of the objects of type <tt>Symbol</tt> representing those non-terminals
on the parse stack.  <tt>RESULT</tt> is of type <tt>Integer</tt> as
well, since the res