described in the next paragraph.  When the mode is
   explicitly disabled (with <code class="code">~with_position:false</code>), the lexing
   engine will not touch the position fields and the lexer actions
   should be careful not to do it either; the <code class="code">lex_curr_p</code> and
   <code class="code">lex_start_p</code> field will then always hold the <code class="code">dummy_pos</code> invalid
   position.  Not tracking positions avoids allocations and memory
   writes and can significantly im