title: How to get a Lookup from nested dictionaries
body: <p>I have a nested dictionary from which I want to derive a Lookup. Example data of the dictionary is following:</p>
  
  <pre><code>var binary_transaction_model = new Dictionary&lt;string, Dictionary&lt;string, bool&gt;&gt;();
  binary_transaction_model.Add("123", new Dictionary&lt;string, bool&gt;(){{"829L", false},{"830L", true}});
  binary_transaction_model.Add("456", new Dictionary&lt;string, bool&gt;(){{"829L", true},{"830L", false}});
  binary_transaction_model.Add("789", new Dictionary&lt;string, bool&gt;(){{"829L", true},{"830L", true}});
  </code></pre>
  
  <p>This LINQ statement is working:</p>
  
  <pre><code>var cols = from a in binary_transaction_model
      from b in a.Value
      where b.Value == true
      group a.Key by b.Key;
  </code></pre>
  
  <p>It gives me an <code>IEnumerable&lt;IGrouping&lt;String,String&gt;&gt;</code>. For lookup purposes I need this result as a Lookup data structure. How can I do this? How should a <code>ToLookup()</code> signature look like? (<strong><em>Edit:</strong> I want to have a <code>Lookup&lt;String,String&gt;</code>.</em>)</p>