added test for negative exponent for simplify_exponential_function

src/test/py/test_read_math_function.py

@@ -85,6 +85,9 @@ class ReadFunction(unittest.TestCase):
     def test_simplify_exponential_function_negative_premultiplicator(self):
         self.assertEqual(simplify_exponential_function("e", "-2x^1"), ["-2"])
 
+    def test_simplify_exponential_function_negative_exponent(self):
+        self.assertEqual(simplify_exponential_function("e", "2x^-2"), ["-4x^-3"])
+
 
 if __name__ == '__main__':
     unittest.main()