ADVANCE
left_end_reached = ADVANCE;
right_end_reached = ADVANCE;
advance_right = ADVANCE;
advance_left = ADVANCE;
advance_left = ADVANCE;
advance_right = ADVANCE;
advance_left = ADVANCE;
advance_right = ADVANCE;
advance_left = ADVANCE;
advance_right = ADVANCE;
advance_left = ADVANCE;
advance_right = ADVANCE;
advance_right = ADVANCE;
advance_left = ADVANCE;