Added advanded unit tests for d/dx e^x.

src/rules/derivatives.py

@@ -197,7 +197,6 @@ def variable_exponent(root, args):
     Note that (in combination with logarithmic/constant rules):
     der(e ^ x)  ->  e ^ x * ln(e)  ->  e ^ x * 1  ->  e ^ x
     """
-    # TODO: Put above example 'der(e ^ x)' in unit test
     g, x = root[0]
 
     return g ** x * ln(g)

tests/test_rules_derivatives.py

@@ -228,3 +228,12 @@ class TestRulesDerivatives(RulesTestCase):
         f, g = root[0]
         self.assertEqual(quotient_rule(root, ()),
                          (der(f) * g - f * der(g)) / g ** 2)
+
+    def test_natural_pase_chain(self):
+        self.assertRewrite([
+            'der(e ^ x)',
+            'e ^ x * ln(e)',
+            'e ^ x * (log(e) / log(e))',
+            'e ^ x * 1',
+            'e ^ x',
+        ])