title: Set multi column index in pandas
body: <p>I make dataframe like this.</p>
  
  <pre><code>df = pd.DataFrame({
      'class' : ['A', 'A', 'A', 'A', 'A', 'B', 'B', 'B', 'B', 'B'],
      'number' : [1,2,3,4,5,1,2,3,4,5],
      'math' : [90, 20, 50, 30, 57, 67, 89, 79, 45, 23],
      'english' : [40, 21, 68, 89, 90, 87, 89, 54, 21, 23]
  })
  </code></pre>
  
  <p>and i want to convert index to this by using some pandas methods.(ex. set_index, stack,,,)</p>
  
  <pre><code>df1 = pd.DataFrame(np.random.randint(1, 100, (5, 4)),
               columns = [['A', 'A', 'B', 'B'],['english', 'math', 'english', 'math']],
               index = [1, 2, 3, 4, 5])
  </code></pre>
  
  <p>how can i do this?</p>