diff --git a/m2cgen/assemblers/xgboost.py b/m2cgen/assemblers/xgboost.py
index 4472f9d8..e83b0150 100644
--- a/m2cgen/assemblers/xgboost.py
+++ b/m2cgen/assemblers/xgboost.py
@@ -77,6 +77,9 @@ def _assemble_tree(self, tree):
         feature_idx = self._feature_name_to_idx[tree["split"]]
         feature_ref = ast.FeatureRef(feature_idx)
 
+        # Since comparison with NaN (missing) value always returns false we
+        # should make sure that the node ID specified in the "missing" field
+        # always ends up in the "else" branch of the ast.IfExpr.
         use_lt_comp = tree["missing"] == tree["no"]
         if use_lt_comp:
             comp_op = ast.CompOpType.LT
diff --git a/m2cgen/interpreters/interpreter.py b/m2cgen/interpreters/interpreter.py
index fa256c9c..3997fa27 100644
--- a/m2cgen/interpreters/interpreter.py
+++ b/m2cgen/interpreters/interpreter.py
@@ -16,6 +16,8 @@ def interpret(self, expr):
         self._reset_reused_expr_cache()
         return self._do_interpret(expr)
 
+    # Private methods implementing Visitor pattern
+
     def _pre_interpret_hook(self, expr, **kwargs):
         return None, kwargs