major cleanup of tsig datastructures and extend/merge operations; fixes old bugs in classes/arities code; proper treatment of nonterminals and syntax-only types;
Sep2 = Main +
consts sep :: 'a list => 'a => 'a list
recdef sep "measure length"
"sep (x#y#zs) = (%a. x # a # sep zs a)"
"sep xs = (%a. xs)"
end