removed get_nodes;
added keys;
all_preds / all_succs: topological order;
added del_nodes;
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)"