datatype none = Nil
datatype ('a,'b, 'c) abctree = Leaf of none*none*none
                        | Br1 of 'a * (('a,'b, 'c) abctree) * (('a,'b, 'c) abctree)
                        | Br2 of 'b * (('b,'b, 'c) abctree) * (('b,'b, 'c) abctree)
                        | Br2 of 'c * (('c,'c, 'c) abctree) * (('c,'c, 'c) abctree)