diff --git a/src/Lib.hs b/src/Lib.hs
index 837103c..8e9a7e5 100644
--- a/src/Lib.hs
+++ b/src/Lib.hs
@@ -102,9 +102,9 @@ updateVariables ds vs = foldl updateVariable (Right vs) ds
 updateVariable :: Either String (Map String Rational) -> (String, Expr) -> Either String (Map String Rational)
 updateVariable (Left  e)  _         = Left e
 updateVariable (Right vs) (name, e) = case updateVariables nvs vs of
-        Left e    -> Left e
+        Left e    -> Left $ "In definition of variable '" ++ name ++ "':\n" ++ e
         Right uvs -> case replaceVars e uvs of
-            Left e    -> Left e
+            Left e    -> Left $ "In definition of variable '" ++ name ++ "':\n" ++ e
             Right exp -> Right $ Map.insert name (evaluate exp) uvs
     where
         nvs = extractVariableDefinitions e
@@ -130,7 +130,9 @@ replaceVars (Binary op l r)               vs = case leftBranch of
 rationalPower :: Rational -> Rational -> Rational
 rationalPower a b = rationalPower' (numerator a, denominator a) (numerator b, denominator b)
     where
-        rationalPower' (a, b) (c, 1) = a ^ c % b ^ c
+        rationalPower' (a, b) (c, 1) 
+            | c >= 0    = a ^ c % b ^ c
+            | otherwise = 1 / rationalPower (a % b) (-c % 1)
         rationalPower' _ _ = error "Powers with unnatural numbers are not supported yet"
 
 evaluate :: Expr -> Rational