title: Group nested list with linq
body: <p>I have a nested list of objects. That I need to group by <code>identifierA</code> and <code>Sum</code> its numeric properties, nested list shall group respectively:</p>
  
  <pre><code>public class TypeA
  {
      public String identifierA{ get; set; }
      public Int32 number { get; set; }
      public List&lt;TypeB&gt; nestedList { get; set; }
  }
  public class TypeB
  {
      public String identifierB { get; set; }
      public Int32 otherNumber { get; set; }
  }
  </code></pre>
  
  <p>So I'm expecting something like this:</p>
  
  <pre><code>var List&lt;TypeA&gt; groupedList = (from a in TypeAList
                                 group a by a.identifierA
                                 into groupedData
                                 select new TypeA
                                 {
                                     identifierA = groupedData.Key,
                                     number = groupedData.Sum(g =&gt; g.number ),
                                     nestedList = //HOW TO GROUP NESTED PART?
                                 }).ToList();
  </code></pre>