renamed kill_all to kill, in conformance with atp_kill command;
simplified/unified treatment of preferences;
check_thread_manager: CRITICAL due to global ref;
goal addressing via Thm.cprem_of;
reduced NJ basis library stuff to bare minimum;
structure Codegen =
struct
val arbitrary_option : 'a option = NONE;
fun dummy_option [] = arbitrary_option
| dummy_option (x :: xs) = SOME x;
end; (*struct Codegen*)