diff --git a/src/Parser/Numbers.md b/src/Parser/Numbers.md
index aa0e156..195570f 100644
--- a/src/Parser/Numbers.md
+++ b/src/Parser/Numbers.md
@@ -51,7 +51,11 @@ hex = MkBase 16
 
 ```idris
 export
-nat : Parser Nat
+nat : Base -> Parser Nat
+
+export
+natBase10 : Parser Nat
+natBase10 = nat base10
 ```
 
 ## Unit tests
@@ -68,7 +72,24 @@ roundtrip x p = do
       putStrLn "Failed to produce parser for \{string}"
       pure False
   Right result <- runEff (rundownFirst p) [handleParserStateIO state] {m = IO}
-    | Left err => ?show_err
+    | Left err => do
+      printLn err
+      pure False
   putStrLn "Input: \{string} Output: \{show result}"
   pure $ x == result
 ```
+
+Do some roundtrip tests with the nat parser
+
+```idris
+-- @@test Nat round trip
+natRoundTrip : IO Bool
+natRoundTrip = pure $ 
+  !(roundtrip 0 natBase10) 
+  && !(roundtrip 1 natBase10) 
+  && !(roundtrip 100 natBase10) 
+  && !(roundtrip 1234 natBase10) 
+  && !(roundtrip 1234567890 natBase10) 
+  && !(roundtrip 1234567890000 natBase10) 
+  && !(roundtrip 12345678901234567890 natBase10) 
+```