copilot-theorem: Look for falsifiable (not invalid). Refs Copilot…

…-Language#495.

In Kind2, disproven properties are tagged with `falsifiable` in the XML output,
but the code in `copilot-theorem`'s Kind2 backend was instead searching for a
tag named `invalid`. As a result, `copilot-theorem` would error when attempting
to disprove properties that are false, as it fail to parse the XML output. This
fixes the issue by replacing `invalid` with `falsifiable`.

copilot-theorem/src/Copilot/Theorem/Kind2/Output.hs

@@ -19,9 +19,9 @@ parseOutput :: String    -- ^ Property whose validity is being checked.
 parseOutput prop xml = fromJust $ do
   root <- parseXMLDoc xml
   case findAnswer . findPropTag $ root of
-    "valid"   -> return (Output Valid   [])
-    "invalid" -> return (Output Invalid [])
-    s         -> err $ "Unrecognized status : " ++ s
+    "valid"       -> return (Output Valid   [])
+    "falsifiable" -> return (Output Invalid [])
+    s             -> err $ "Unrecognized status : " ++ s
 
   where