origin = ( 2.0,8 )
	cht = 1.1*cht; cwid = 1.25*cwid;
	B1:	box
		arrow;  box;  arrow
	B3:	box
		arc -> from top of B3 to top of B1
	move to ( 2,3.5 )
	arrow
	box ht 0.3 wid 0.3 'x'
	box same
	arrow from center of last box
	box same 'y'
	box same
	arrow from center of last box
	box same 'z'
	box same
	line from last box.sw to last box.ne