 mapT maps a change function to the result of a transformation
-- (mapT is somehow a lifted form of mapD)
-- The restricted type of f results from the fact that the
-- argument to t cannot be changed to b in the result Trans type.
-- 
mapT :: Change a -> Trans a -> Trans a
mapT f t = mapD f . t


mapT :: Change a -> Trans a -> Trans a
mapT f t = mapD f . t
mapT f t = mapD f . t