title: Identify NA's in sequence row-wise
body: <p>I want to fill NA values in a sequence, which is row-wise, based on a condition. Please see example below. </p>
  
  <pre><code>ID | Observation 1 | Observation 2 | Observation 3 | Observation 4 | Observation 5
   A         NA              0               1             NA             NA
  </code></pre>
  
  <p>The condition is: </p>
  
  <ul>
  <li>all NA values before !NA values in the sequence should be left as NA; </li>
  <li>but all NAs after !NA values in the sequence should be tagged ("remove")</li>
  </ul>
  
  <p>In the example above, NA value in Observation 1 should remain NA. However, the NA values in Observations 4 and 5 should be changed to "Remove". </p>