test_remove_value = function()
	local h = {1,2,3}
	table.remove_value(h, 1)
	check_eq(h, {2,3}, "a")
	local h = {1,2,3}
	table.remove_value(h, 2)
	check_eq(h, {1,3}, "b")
	local h = {1,2,3}
	table.remove_value(h, 3)
	check_eq(h, {1,2}, "b")
	local h = {1,2,3}
	table.remove_value(h, 4)
	check_eq(h, {1,2,3}, "b")
end

table.remove_value = function(h, val)
	local i = table.find(h, val)
	if i then
		table.remove(h, i)
	end
end

detach_edge = function(edge)
	local src, dest = edge[1], edge[2]
	table.remove_value(Nodes[src].outgoing_edges, dest)
	table.remove_value(Nodes[dest].incoming_edges, src)
end

on_edge = function(sx,sy)
	-- check if mouse is on the border of a node and also on an edge to another node.
	-- reading from Surface; I'm going to regret this.
	for _,shape in ipairs(Surface) do
		if shape.type == 'line' then
			if shape.keys then
				-- it's an edge; assuming keys are in a specific, brittle order
				if distance_sq(sx,sy, shape.data[1], shape.data[2]) < 5*5 then
					return shape.keys
				end
				if distance_sq(sx,sy, shape.data[3], shape.data[4]) < 5*5 then
					return shape.keys
				end
			end
		end
	end
end

	elseif on_edge(sx,sy) then
		set_mouse_cursor('no')

		Edge = {source=node, s={sx=sx, sy=sy}, e={sx=sx, sy=sy}}

		local edge = on_edge(sx,sy)
		if edge == nil then
			Edge = {source=node, s={sx=sx, sy=sy}, e={sx=sx, sy=sy}}
		else
			detach_edge(edge)
			A()
		end

end

end