title: How to create a group from a group in crossfilter/reductio?
body: <p>With the following data:</p>
  
  <pre><code>const now = new Date
  const data = [
    { player: 'bob', color: 'blue', date: new Date(+now + 1000) },
    { player: 'bill', color: 'green', date: new Date(+now + 2000) },
    { player: 'bob', color: 'red', date: new Date(+now + 3000) },
    { player: 'barbara', color: 'blue', date: new Date(+now + 4000) },
    { player: 'barbara', color: 'cyan', date: new Date(+now + 8000) },
    { player: 'barbara', color: 'magenta', date: new Date(+now + 10000) },
    { player: 'barbara', color: 'yellow', date: new Date(+now + 20000) },
  ]
  </code></pre>
  
  <p>I want to reduceCount on the color dimension, but <em>only</em> count the first color per-player. <em>(note: first is w.r.t. the date dimension which may be filtered).</em> I have been trying to get this to work with reductio using the exception feature but it did not give the expected results:</p>
  
  <pre><code>reducer = reductio()
  reducer.exception('player').exceptionCount(true)
  reducer(colorGroup)
  </code></pre>
  
  <p>The results should look like this:</p>
  
  <pre><code>blue,2     # bob and barbara
  cyan,0
  green,1    # bill
  magenta,0
  red,0
  yellow,0
  </code></pre>
  
  <p>Another example, with the date dimension filtered to <code>now+2100</code>..<code>now+20000</code> (i.e. the first row is filtered out):</p>
  
  <pre><code>blue,1     # barbara
  cyan,0
  green,0    # (bill's first color is not counted because it is outside the date range)
  magenta,0
  red,1      # bob (blue is his first color overall, red is his first in this date range)
  yellow,0
  </code></pre>
  
  <p>Note: I have other groupings which use all of the rows so I can't just pre-filter the list before loading it into crossfilter.</p>
  
  <p>Is there some way to use reductio() for this? Or some example of how to do this "grouping upon grouping" with crossfilter directly?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>Link to jsfiddle showing unexpected result: <a href="https://jsfiddle.net/qt5jxjm1/" rel="nofollow">https://jsfiddle.net/qt5jxjm1/</a></p>