title: Delete all leaves in a tree in Haskell
body: <p>Hey guys i have a problem. I would like to delete all leaves of a tree.
  My init of a tree is <code>data NBaum a = NBlatt a | NKnoten a [NBaum a]</code>. I have no idea how i can do this and which is the command that the Nblatt is empty. Maybe u guys can help me.</p>
  
  <p>My function should take a NBaum as an input and the output is a NBaum too.<code>entferneBlaetter :: NBaum a -&gt; NBaum a</code></p>
  
  <p>I tried already  this code but it's wrong:</p>
  
  <pre><code>entferneBlaetter (NBlatt _) = NBlatt "test"
  entferneBlaetter (NKnoten a b) = NKnoten a (entferneBlaetter_help b)
  </code></pre>