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 
`['feature_one', 'feature_two', 'feature_three']`, then -2 would refer to `feature_two`. I am surprised with usage of negative index. In would make more sense to refer to `feature_two` by index 1. (-2 is reference convenient for human digestion, not for machine processing). Am I reading it correctly?

**Update:** Here is an example:

```
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)

```

Here are the files [X](https://www.dropbox.com/s/mh3zdxsp0g37mkw/X.csv?dl=0) and [Y](https://www.dropbox.com/s/4sijljgmz46f6hc/Y.csv?dl=0)

Here is the output:

```
    [ 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]

```