In some folds over data when creating ocmalgraph's, I'd need an empty  
vertex value, something like V.null, to give to an edge-creating fold  
when I know it's not going to be used.  Currently I have to look ahead  
into the data.  Does it make sense to add a null value to the V  
module, or is there another OCaml way to give a fold an empty V value?

Cheers,
Alexy