removed separate logtypes field of syntax; removed test_read, simple_str_of_sort, simple_string_of_typ; provide default_mode;
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)"