U_W_AOF
mode &= ~(U_R_VAL | U_W_AOF);
mode &= ~U_W_AOF;
mode |= U_W_AOF;
do_expression(U_W_AOF | mode, expr));
? U_R_AOF | (mode & U_W_AOF) : 0;
? U_W_AOF | U_R_VAL : U_R_VAL;