title: Create a set of numpy arrays
body: <p>I want to create a set of numpy arrays and then efficiently perform the <code>in</code> operation.</p>
  
  <pre><code>explored = {
         np.array([[4,9,6,1,2,5], [5,8,3,7,7,9], [2,4,1,6,3,8]]),
         np.array([[1,2,3,1,2,5], [1,2,3,7,7,9], [1,2,3,6,3,8]]),
             }
  
  if array in explored: # where array is an np.array 
      print("the array is already in the set")
  else:
      explored.append(array)
  </code></pre>
  
  <p>But the interpreter complains that <code>np.array</code> is not a hashable type. What can I do instead?</p>