http://www.plattegronden.nl/cgi-local/imap.cgi?hoorn+hoor+g+5+e+4+e+3 - Show boxes e3 and e4; the arrows are next to e3, the current box; applying one retains that box, and replaces e4 by the box in the specified direction from e3 and makes that the current box. Applying the arrow toward the other box just removes e3.