title: How to group by index and index+1 in Pig
body: <p>I have data input like this:</p>
  
  <pre><code>(index,x,y)
  (1,0.0,0.0)
  (2,-0.1,-0.1)
  (3,1.0,-2.2)
  ...
  </code></pre>
  
  <p>how can I group by [index] and [index + 1] like</p>
  
  <pre><code>{(1, 0.0, 0.0), (2, -0.1, -0.1)}
  {(2, -0.1, -0.1), (3, 1.0, -2.2)}
  ...
  </code></pre>
  
  <p>Please help me through this. Thanks.</p>