title: C# Linq Sum query on IDictionary
body: <p>I have an <code>IDictionary&lt;string,int&gt;</code> containing a distinct list of categories and respective counts with data looking something like this:</p>
  
  <pre>
  Category  |  Count
  -------------------
  A         |    2
  B         |    2
  Z         |    2
  A_B       |    1
  A_B_C     |    5
  </pre>
  
  <p>I'm looking for a query that sums the count of categories that start with common letters/string, so from the above the results I'm looking for is as follows:</p>
  
  <pre>
  Category  |  Count
  -------------------
  A         |    8     &lt;- (a sum of A, A_B, A_B_C)
  B         |    2
  Z         |    2
  A_B       |    6     &lt;- (a sum of A_B, A_B_C)
  A_B_C     |    5
  </pre>