backtrace_object  533 vm_backtrace.c     return backtrace_object(GET_THREAD());
backtrace_object  662 vm_backtrace.c     return backtrace_to_str_ary(backtrace_object(th), lev, n);
backtrace_object  668 vm_backtrace.c     return backtrace_to_location_ary(backtrace_object(th), lev, n);
backtrace_object  822 vm_backtrace.c     VALUE btval = backtrace_object(th);