isBound :: Env -> String -> IO Bool
isBound envRef var = readIORef envRef >>= return . maybe False (const True) . lookup var
isBound :: Env -> String -> IO Bool
isBound envRef var = readIORef envRef >>= return . maybe False (const True) . lookup var
isBound envRef var = readIORef envRef >>= return . maybe False (const True) . lookup var