Some tuning:
- finite now uses rev_append (tail recursive!) to append stopper, because @
needs to much stack space for large strings
- repeat is now tail recursive
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)"