thenNat :: NatM a -> (a -> NatM b) -> NatM b
thenNat expr cont
        = NatM $ \st -> case unNat expr st of
                        (result, st') -> unNat (cont result) st'
thenNat :: NatM a -> (a -> NatM b) -> NatM b
thenNat expr cont
        = NatM $ \st -> case unNat expr st of
                        (result, st') -> unNat (cont result) st'
thenNat expr cont
        = NatM $ \st -> case unNat expr st of
                        (result, st') -> unNat (cont result) st'