eSpot(int idx) or computeSpot(long time)</li>
<li>It can correspond to the Options change (Options are transfered in by the caller as reference)</li>
<li>It can remember the status intermedial of function instance to avoid redundant computation. The status includes</li>
  <ul>
    <li>backward values (as it's a TimeSeries's variable)</li>
    <li>the based Variables</li>
	<li>the Options</li>
	<li>the caller</li>
  </ul>
</ul>

<p>

Then an indicator instance can call it to:
<ul>
<li>compute from 'fromIdx' 