sign = function(n)
	if n < 0 then return -1
	else return 1 end
end

angle = function(x1, y1, x2, y2)
	return math.atan((y2-y1)/(x2-x1))
end

	table.insert(Surface, {type='rectangle', r=0,g=0,b=0, x=e2.sx-2, y=e2.sy-2, w=4, h=4, keys={s,e}})
end

	-- arrow head
	local angle = angle(e2.sx, e2.sy, s2.sx, s2.sy)
	-- e2.sx + D*cos(angle) == s2.sx
	-- D = (s2.sx - e2.sx)/cos(angle)
	-- draw the arrow head with the same sign as D
	local arrow_dir = sign((s2.sx-e2.sx)/(math.cos(angle)))
	local arrow_len = 10*arrow_dir
	local d = math.pi/10
	table.insert(Surface, {type='line', r=0,g=0,b=0, keys={s,e}, data={
		e2.sx, e2.sy,
		e2.sx+arrow_len*math.cos(angle+d),
		e2.sy+arrow_len*math.sin(angle+d),
	}})
	table.insert(Surface, {type='line', r=0,g=0,b=0, keys={s,e}, data={
		e2.sx, e2.sy,
		e2.sx+arrow_len*math.cos(angle-d),
		e2.sy+arrow_len*math.sin(angle-d),
	}})
end