Fixed two small print bugs

src/Main.hs

@@ -305,7 +305,7 @@ checkSafetyProperty verbosity net refine f traps = do
                     rt <- checkSat $ checkTrapSat net assigned
                     case rt of
                         Nothing -> do
-                            putStrLn "No trap found."
+                            verbosePut verbosity 1 "No trap found."
                             return False
                         Just at -> do
                             let trap = trapFromAssignment at

src/Printer.hs

@@ -5,6 +5,6 @@ where
 import Data.Char
 
 validateId :: String -> String
-validateId "" = error "empty id"
+validateId "" = ""
 validateId (x:xs) = (if isAlpha x then x else '_') :
         map (\c -> if isAlphaNum c then c else '_') xs