Some code coverage fixes.

src/rules/derivatives.py

@@ -155,23 +155,20 @@ def match_variable_power(node):
     evars = find_variables(exponent)
     x = get_derivation_variable(node, rvars | evars)
 
-    if x in rvars and x in evars:
+    if x in rvars:
+        if x in evars:
             return [P(node, power_rule)]
 
-    if x in rvars:
         if root.is_variable():
             return [P(node, variable_root)]
 
         return [P(node, chain_rule, (root, variable_root, ()))]
 
-    if x in evars:
     if exponent.is_variable():
         return [P(node, variable_exponent)]
 
     return [P(node, chain_rule, (exponent, variable_exponent, ()))]
 
-    return []
-
 
 def power_rule(root, args):
     """

tests/test_rules_integrals.py

@@ -132,13 +132,20 @@ class TestRulesIntegrals(RulesTestCase):
         ])
 
     def test_match_function_integral(self):
-        root0, root1, root2 = tree('int ln x, int sin x, int cos x')
-        self.assertEqualPos(match_function_integral(root0),
-                [P(root0, logarithm_integral)])
-        self.assertEqualPos(match_function_integral(root1),
-                [P(root1, sinus_integral)])
-        self.assertEqualPos(match_function_integral(root2),
-                [P(root2, cosinus_integral)])
+        root = tree('int ln x')
+        self.assertEqualPos(match_function_integral(root),
+                [P(root, logarithm_integral)])
+
+        root = tree('int sin x')
+        self.assertEqualPos(match_function_integral(root),
+                [P(root, sinus_integral)])
+
+        root = tree('int cos x')
+        self.assertEqualPos(match_function_integral(root),
+                [P(root, cosinus_integral)])
+
+        root = tree('int sqrt x')
+        self.assertEqualPos(match_function_integral(root), [])
 
     def test_logarithm_integral(self):
         root, expect = tree('int ln x, (xlnx - x) / ln e + c')