added METHOD_CASES, resolveq_cases_tac;
removed multi_resolveq;
improved 'tactic' method: bind thm(s) function;
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)"