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.
consts valif :: ifex => (nat => bool) => bool
primrec
"valif (CIF b) env = b"
"valif (VIF x) env = env x"
"valif (IF b t e) env = (if valif b env then valif t env
else valif e env)"