title: How to calculate weighted average in window join?
body: <p>Is there a way to calculate weighted average in a window join (wj)
  Since we can use <code>min, max, avg</code> there should be a way to use <code>wavg</code> too? </p>
  
  <pre><code>wj[w; `sym`time; tbl; (data; (wavg; `quantity; `price))]
  </code></pre>
  
  <p>This just gives me a list of values preceded by wavg</p>