A1 = function(key)
	-- like A, but updating a single node
	-- this only works because Nodes is a flat array; we don't support row/col types here yet
	-- delete previously added shapes for this node:
	for i=#Surface,1,-1 do
		local x = Surface[i]
		if x.key == key then
			table.remove(Surface, i)
		end
	end	
	local node = Definitions[key]
	compute_layout(node, node.x,node.y, Surface)
	-- continue the pipeline
	B()
end