title: diagonally reflect a matrix
body: <p>I'm not sure if this question has been either asked
  or quite possibly already answered. If I start with
  an original 3x3 matrix:</p>
  
  <pre>
      1 2 3
      4 5 6
      7 8 9
  </pre>
  
  <p>, how would I produce the following 3x3 matrix:</p>
  
  <pre>
      9 6 3
      8 5 2
      7 4 1 
  </pre>
  
  <p>??</p>