title: What is the output of clf.tree_.feature?
body: <p>I observed that scikit-learn clf.tree_.feature occasional return negative values. For example -2. As far as I understand clf.tree_.feature is supposed to return sequential order of the features. In case we have array of feature names 
  <code>['feature_one', 'feature_two', 'feature_three']</code>, then -2 would refer to <code>feature_two</code>. I am surprised with usage of negative index. In would make more sense to refer to <code>feature_two</code> by index 1. (-2 is reference convenient for human digestion, not for machine processing). Am I reading it correctly?</p>
  
  <p><strong>Update:</strong> Here is an example:</p>
  
  <pre><code>def leaf_ordering():
      X = np.genfromtxt('X.csv', delimiter=',')
      Y = np.genfromtxt('Y.csv',delimiter=',')
      dt = DecisionTreeClassifier(min_samples_leaf=10, random_state=99)
      dt.fit(X, Y)
      print(dt.tree_.feature)
  </code></pre>
  
  <p>Here are the files <a href="https://www.dropbox.com/s/mh3zdxsp0g37mkw/X.csv?dl=0" rel="noreferrer">X</a> and <a href="https://www.dropbox.com/s/4sijljgmz46f6hc/Y.csv?dl=0" rel="noreferrer">Y</a></p>
  
  <p>Here is the output:</p>
  
  <pre><code>    [ 8  9 -2 -2  9  4 -2  9  8 -2 -2  0  0  9  9  8 -2 -2  9 -2 -2  6 -2 -2 -2
    2 -2  9  8  6  9 -2 -2 -2  8  9 -2  9  6 -2 -2 -2  6 -2 -2  9 -2  6 -2 -2
    2 -2 -2]
  </code></pre>