TIP_SWITCH
__u8 tip_switch = data[1] & TIP_SWITCH;
__u8 tip_held = last_button_state & TIP_SWITCH;
data[1] &= ~(TIP_SWITCH | BARREL_SWITCH); /* release tip and barrel */
prev_tip = !!(prev_state & TIP_SWITCH);
if ((changed_state & (TIP_SWITCH | ERASER)) == (TIP_SWITCH | ERASER)) /* we get both a tipswitch and eraser change at the same time */