added test for no 'x' in function for difference_of_simplified_exponential_functions

src/test/py/test_read_math_function.py

@@ -97,6 +97,9 @@ class ReadFunction(unittest.TestCase):
     def test_sum_of_simplified_exponential_functions_no_x(self):
         self.assertEqual(sum_of_simplified_exponential_functions("se", "2^1+2^1"), "Der Ausdruck muss die Variable 'x' enthalten.")
 
+    def test_difference_of_simplified_exponential_functions_no_x(self):
+        self.assertEqual(difference_of_simplified_exponential_functions("de", "2^1-2^1"), "Der Ausdruck muss die Variable 'x' enthalten.")
+
 
 if __name__ == '__main__':
     unittest.main()