title: How would you unionize N-arrays with different sizes?
body: <p>The :</p>
  
  <pre><code> np.union1d(a, b)
  </code></pre>
  
  <p>can unionize two arrays with different sizes.</p>
  
  <pre><code> np.vstack((a, b, c)).T.ravel()
  </code></pre>
  
  <p>can unionize N arrays of the same size.</p>
  
  <p>How would you unionize N-arrays with different sizes ?
  And of course it should be fast ;) !</p>
  
  <hr>
  
  <p>btw union is not just concatenation...
  still testing, but would this do it :</p>
  
  <pre><code> np.unique(np.concatenate((a,b,c)))
  </code></pre>