e (i.e. setting it to 100 will double the heap
        size at each increase). If it is more than 1000, it is a fixed
        number of words that will be added to the heap. Default: 15.<br>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code>&nbsp;&nbsp;</code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable&nbsp;</span><span id="TYPEELTcontrol.space_overhead">space_overhead</span>&nbsp;: <code