equence.</p><p>

The error symbol only comes into play if a syntax error is
detected.  If a syntax error is detected then the parser tries to replace
some portion of the input token stream with <tt>error</tt> and then
continue parsing.  For example, we might have productions such as:

</p><pre><tt>    stmt ::= expr SEMI | while_stmt SEMI | if_stmt SEMI | ... |
	     error SEMI
	     ;</tt></pre>

This indicates that if none of the normal productions for <tt>stmt</tt> can
be matched by the input, then a synt