Add negative as derived features

dtree_learner.py

@@ -78,10 +78,16 @@ class DTreeLearner(LearnerBase):
     @staticmethod
     def _generate_derived_features(
             base_vars: List[str], k: int = 2) -> List[Tuple[str, Tuple[Any, str]]]:
+        res = []
+        for var in base_vars:
+            var_coeff_map = {var: -1}
+            expr = f"(-1*{var})"
+            name = expr
+            res.append((name, (var_coeff_map, expr)))
+
         if len(base_vars) < k:
-            return []
+            return res
 
-        res = []
         coeff_combinations = list(itertools.product([1, -1], repeat=k))
         var_id_iter = range(len(base_vars))
         for selected_var_ids in itertools.combinations(var_id_iter, k):