Use smtlib2 "user" variable names starting with "s" instead of "x".

saw-core-sbv/src/Verifier/SAW/Simulator/SBV.hs

@@ -678,7 +678,7 @@ nextId :: StateT Int IO String
 nextId = ST.get >>= (\s -> modify (+1) >> return ("x" ++ show s))
 
 nextId' :: String -> StateT Int IO String
-nextId' nm = nextId <&> \s -> s ++ "_" ++ nm
+nextId' nm = ST.get >>= (\s -> modify (+1) >> return ("s" ++ show s ++ "_" ++ nm))
 
 unzipMap :: Map k (a, b) -> (Map k a, Map k b)
 unzipMap m = (fmap fst m, fmap snd m)