major cleanup of tsig datastructures and extend/merge operations; fixes old bugs in classes/arities code; proper treatment of nonterminals and syntax-only types;
consts normal :: ifex => bool
primrec
"normal(CIF b) = True"
"normal(VIF x) = True"
"normal(IF b t e) = (normal t & normal e &
(case b of CIF b => True | VIF x => True | IF x y z => False))"