Multipartite or layered graph

Hi, I need to implement a graph whose set of vertices is partitioned into a certain number of ordered subsets (called layers). Edges join vertices from a layer to the next one(s). A critical operation is the iteration over the nodes of a given layer. I think this can be implemented by adding a vertex property to each vertex, that specifies the layer each vertex belongs to. But this does not seem to me the most (time and space) efficient solution: in particular, how can I iterate over the nodes of a given layer without checking every node? I was wondering whether there is a better way to do the job: maybe, using a custom VertexList implemented using a vector for each layer (in this case, should I write my own functions to add/remove vertices? And should I write my own iterators?). Any other idea, or pointer to existing code (I am new to Boost)? Best regards Nicola Vitacolonna