major cleanup of tsig datastructures and extend/merge operations; fixes old bugs in classes/arities code; proper treatment of nonterminals and syntax-only types;
Fib = WF_Rel +
consts fib :: nat => nat
recdef fib "measure(%n. n)"
"fib 0 = 0"
"fib 1 = 1"
"fib (Suc(Suc x)) = fib x + fib (Suc x)"
end