MSG_PREV_PAGE
fPrev = new BButton(bounds, "previous", "Previous page", new BMessage(MSG_PREV_PAGE));
case MSG_PREV_PAGE:
fPrev = new BButton(BRect(left, top, left+10, top+10), "Prev", "Previous Page", new BMessage(MSG_PREV_PAGE));
case MSG_PREV_PAGE: fPreview->ShowPrevPage();