title: How to create a new variable by pasting one variable below the other?
body: <p>Fellow statisticians/programmers,</p>
  
  <p>I have a simple problem. I have two variables, x1 and x2 and I want to combine these to to get the variable x3, as below:</p>
  
  <pre><code>x1   x2    x3
  
  1    5     1
  2    4     2
  3    4     3
  1    6     1
             5
             4
             4
             6
  </code></pre>
  
  <p>Probably this is just a simple function, but I really cannot find the right command. </p>
  
  <p>Can somebody please help me? Many thanks in advance!</p>