title: Sum column B until there is any change in column A
body: <p>I'm trying to create a formula that does a sum of column B until there is any change in column A. 
  E.g.</p>
  
  <pre><code>    Col A   Col B
  
  1     X       2
  2     X       1
  3     X       0
  4     X       2
  5     Y       1
  </code></pre>
  
  <p>Starting at the point of A1, this would then show the sum of 5. Column A in this situation would always be sorted (X, followed by Y, followed by Z etc.)</p>