id_bt             705 error.c            rb_ivar_set(exc, id_bt, Qnil);
id_bt             831 error.c            obj = rb_attr_get(exc, id_bt);
id_bt             898 error.c            return rb_ivar_set(exc, id_bt, rb_check_backtrace(bt));
id_bt            1229 error.c            rb_ivar_set(exc, id_bt, Qnil);