-- give_to is a many-to-many relation
-- receive_from inverts the relation give_to
-- The pattern produces the inverse by narrowing

import Control.Search.SetFunctions

data BloodTypes = Ap | An | ABp | ABn | Op | On | Bp | Bn
                deriving (Eq, Ord)

-- donor `give_to` receipient
give_to Ap  = Ap ? ABp
give_to Op  = Op ? Ap ? Bp ? ABp
give_to Bp  = Bp ? ABp
give_to ABp = ABp
give_to An  = Ap ? An ? ABp ? ABn
give_to On  = Ap ? An ? ABp ? ABn ? Op ? On ? Bp ? Bn
give_to Bn  = Bp ? Bn ? ABp ? ABn
give_to ABn = ABp ? ABn

receive_from (give_to x) = x

give_to'set x = set1 give_to x
receive_from'set x = set1 receive_from x

-- all the donors for ABn
main = sortValues (receive_from'set ABn)