title: How to grasp the data type Pair in dolphindb?
body: <p>According to the manual of dolphindb,the range is upper bound exclusive of Pair,but when I test this data type:</p>
  
  <pre><code>print a[1:3];
  for(s in 1:3){print s};          
  print  3 in 1:3;
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>[1,2]
  1
  2
  1
  </code></pre>
  
  <p>I think "upper bound exclusive" means open interval at the upper bound, why does <code>3 in 1:3</code> return 1?  How to grasp it?</p>