code:

dpSwitch :: Functor col =>
    (forall sf . (a -> col sf -> col (b, sf)))
    -> col (SF b c)
    -> SF (a, col c) (Event d)
    -> (col (SF b c) -> d -> SF a (col c))
    -> SF a (col c)
dpSwitch rf sfs0 sfe0 k = SF {sfTF = tf0}
    where
	tf0 a0 =
	    let bsfs0 = rf a0 sfs0
		sfcs0 = fmap (\(b0, sf0) -> (sfTF sf0) b0) bsfs0
		cs0   = fmap snd sfcs0
	    in
		(case (sfTF sfe0) (a0, cs0) of
		     (sfe, NoEvent)  -> dpSwitchAux (fmap fst sfcs0) sfe
		     (_,   Event d0) -> fst (sfTF (k sfs0 d0) a0),
	         cs0)

	dpSwitchAux sfs (SFArr _ (FDC NoEvent)) = parAux rf sfs
	dpSwitchAux sfs sfe = SF' tf -- False
	    where
		tf dt a =
		    let bsfs  = rf a sfs
			sfcs' = fmap (\(b, sf) -> (sfTF' sf) dt b) bsfs
			cs    = fmap snd sfcs'
		    in
			(case (sfTF' sfe) dt (a, cs) of
			     (sfe', NoEvent) -> dpSwitchAux (fmap fst sfcs')
							    sfe'
			     (_,    Event d) -> fst (sfTF (k (freezeCol sfs dt)
							     d)
							  a),
                         cs)