Explain the meaning of X_m in modules/tree doc.
@@ -480,6 +480,8 @@ and Misclassification
H(X_m) = 1 - \max(p_{mk})
+where :math:`X_m` is the training data in node :math:`m`
+
Regression criteria
-------------------
@@ -493,6 +495,7 @@ criterion to minimise is the Mean Squared Error
H(X_m) = \frac{1}{N_m} \sum_{i \in N_m} (y_i - c_m)^2
+where :math:`X_m` is the training data in node :math:`m`
.. topic:: References:
