title: Given y_pred with categories 0,1,2 how do I select only categories 1 and 2 to perform metrics on in Keras?
body: <p>Given the confusion matrix:</p>
  
  <pre><code>&gt;  0   1   2
  &gt;0 10  3   4
  &gt;1 2   5   1
  &gt;2 8   5   9
  </code></pre>
  
  <p>how would you write an accuracy metric with the Keras backend that would measure accuracy of the following subsection only?:</p>
  
  <pre><code>&gt;  1   2
  &gt;1 5   1
  &gt;2 5   9
  </code></pre>