__arg_track_join
struct arg_track new_val = __arg_track_join(old, out);
result = __arg_track_join(result, at_stack_out[slot]);
at_stack_out[slot] = __arg_track_join(at_stack_out[slot], new_val);
at_stack_out[slot] = __arg_track_join(at_stack_out[slot], new_val);
at_stack[i] = __arg_track_join(at_stack[i], none);
at_stack[i] = __arg_track_join(at_stack[i], none);