prove the performance of the lexer
   in contexts where <code class="code">lex_start_p</code> and <code class="code">lex_curr_p</code> are not needed.</p>

<p>Position tracking mode works as follows.  At each token, the lexing
   engine will copy <code class="code">lex_curr_p</code> to <code class="code">lex_start_p</code>, then change the
   <code class="code">pos_cnum</code> field of <code class="code">lex_curr_p</code> by updating it with the number of
   characters read since the start of the <code clas