Removing the datatype declaration of "order" allows the standard General.order
to be used. Thus we can use Int.compare and String.compare instead of the
slower home-grown versions.
types 'v binop = 'v => 'v => 'v
datatype ('a,'v) expr = Cex 'v
| Vex 'a
| Bex ('v binop) (('a,'v) expr) (('a,'v) expr)