title: filter must be a piece of code to indicate the filtering logic reports for DolphinDB reactive state engine
body: <p>The script used to create the reactive state engine is:</p>
  <pre><code>priceFilter = [&lt;TradePrice &gt; 0&gt;,&lt;prev(TradePrice) != TradePrice&gt;]
  priceFltEngine = createReactiveStateEngine(name=&quot;price_filter&quot;, metrics=[&lt;MDTime&gt;,factor1], dummyTable=objByName(`trade), outputTable=result, keyColumn=&quot;HTSCSecurityId&quot;, filter=priceFilter, keepOrder=true)
  </code></pre>
  <p>Is it because filter only supports one filtering condition?</p>