title: filter linq lookup based on values
body: <p>I would like to filter a linq Lookup based on its values:</p>
  
  <p>the lookup:</p>
  
  <pre><code>ILookup&lt;int, Article&gt; lookup
  </code></pre>
  
  <p>here's what I've got so far which isn't working:</p>
  
  <pre><code>IList&lt;int&gt; cityIndexes = GetCityIndexesByNames(cities);    
  
  lookup = lookup
                  .Where(p =&gt; p.Any(x =&gt; cityIndexes.Contains((int)x.ArticleCity)))
                  .SelectMany(l =&gt; l)
                  .ToLookup(l =&gt; (int)l.ArticleParentIndex, l =&gt; l);
  </code></pre>
  
  <p>just to clarify: I want to get all the articles with a city index that is contained in the above city index list.</p>