doc-src/Tutorial/Ifexpr/norm

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

consts norm :: ifex => ifex
primrec
"norm (CIF b)    = CIF b"
"norm (VIF x)    = VIF x"
"norm (IF b t e) = normif b (norm t) (norm e)"