major cleanup of tsig datastructures and extend/merge operations; fixes old bugs in classes/arities code; proper treatment of nonterminals and syntax-only types;
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end