title: Vim eats all my lines if I map ddkP
body: <p>In Vim, I set a mapping to move a line up with each press of <code>_</code></p>
  
  <p><code>nnoremap _ ddkP</code></p>
  
  <p>In my experiments, when I reach the top of my Vim file, all the lines get eaten up if I continue to press and hold <code>_</code></p>
  
  <p>Is something wrong? I'm on the Mac, but using the console Vim, not MacVim.</p>