removed prs / prs_fn (broken, because it did not include \n in its
semantics, forcing writeln to add one uncoditionally);
replaced prs_fn by writeln fn;
consts bool2if :: boolex => ifex
primrec
"bool2if (Const b) = CIF b"
"bool2if (Var x) = VIF x"
"bool2if (Neg b) = IF (bool2if b) (CIF False) (CIF True)"
"bool2if (And b c) = IF (bool2if b) (bool2if c) (CIF False)"