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 comp :: ('a,'v) expr => (('a,'v) instr) list
primrec
"comp (Cex v) = [Const v]"
"comp (Vex a) = [Load a]"
"comp (Bex f e1 e2) = (comp e2) @ (comp e1) @ [Apply f]"