expand_proof now also takes an optional term describing the proposition
of the theorem to be expanded (to avoid problems with different theorems
having the same names).
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)"