Add tests of index selection

test/test.py

@@ -203,6 +203,12 @@ class TestBest(unittest.TestCase):
 
     def test_best(self):
         self.assertEqual(self.model.sympy(), sympy.cos(sympy.Symbol("x0")) ** 2)
+    
+    def test_index_selection(self):
+        self.assertEqual(self.model.sympy(-1), sympy.cos(sympy.Symbol("x0")) ** 2)
+        self.assertEqual(self.model.sympy(2), sympy.cos(sympy.Symbol("x0")) ** 2)
+        self.assertEqual(self.model.sympy(1), sympy.cos(sympy.Symbol("x0")))
+        self.assertEqual(self.model.sympy(0), 1.0)
 
     def test_best_tex(self):
         self.assertEqual(self.model.latex(), "\\cos^{2}{\\left(x_{0} \\right)}")