

<BlockQuote>
<DL>
<DT>No production for X </DT>

<DD>The nonterminal X has been used but there is no production for it.
</DD>

<DT>X cannot be reached </DT>

<DD>There is a production for nonterminal X, but X cannot be derived from
the start symbol. </DD>

<DT>X cannot be derived to terminals </DT>

<DD>For example, if there is a production X = &quot;(&quot; X &quot;)&quot; .
</DD>

<DT>X --&gt; Y, Y --&gt; X </DT>

<DD>X and Y are nonterminals with circular derivations. </DD>

<DT>Tokens X and Y cannot b