title: find discontinuity in data using awk
body: <p>Using awk, how to find out the discontinuity in series of data.
  I have got data like below.</p>
  
  <p>Example:
  Input data:</p>
  
  <pre><code>shotline   shot
  1001       1001
  1001       1002
  1001       1003
  1001       1005
  1001       1006
  1001       1009
  1002       1001
  1002       1003
  </code></pre>
  
  <p>Data should be:</p>
  
  <p>Missing data points:</p>
  
  <pre><code>1001     1004
  1001     1007
  1001     1008
  1002     1002
  </code></pre>