title: How to find the first appearance based on certain criteria
body: <p>I have the following proxy data and I am using data table.</p>
  
  <pre><code>Id         V1       V2
  Aa         10      1000
  Aa          20       90
  Ab          20       90
  Ab          20       90
  Ab          20       100
  As          50       90
  As          300     150
  As           150     0
  </code></pre>
  
  <p>I want to create another field to mark 1 for the first appearance of each ID in V2 that is greater than 0. Ie see ID Ab, it only gets marked 1 on its second entry.</p>
  
  <p>So like the following:</p>
  
  <pre><code>Id         V1       V2          Indicator
  Aa         10      1000           1
  Aa          20       90              0
  Ab          20       0                0
  Ab          20       90.             1
  Ab          20       100.           0
  As          50       90.             1
  As          300     150.           0
  As           150     0.              0
  </code></pre>