instance MonadPlus IO => MonadPlus CoreM where
mzero = CoreM (const mzero)
m `mplus` n = CoreM (\rs -> unCoreM m rs `mplus` unCoreM n rs)
instance MonadPlus IO => MonadPlus CoreM where
mzero = CoreM (const mzero)
m `mplus` n = CoreM (\rs -> unCoreM m rs `mplus` unCoreM n rs)