[igraph] nb of links of the bipartite graph of two given subgraphs

[sergiu netotea]

Hi,

What is the fastest test in igraph to check if two vertices are
linked? I ask because I have two vertex sets and I want to count
something like the number of links between the two sets. Perhaps one
could call such a monster the bipartite graph of two given subgraphs.
Or is there already a way to compute such a graph directly, eg by a
function call or a number of function calls, like graph
unions/intersections? I understand there is no support for bipartite
graphs yet, but in this setup the the so called bipartite graph would
be a subgraph itself, of the big graph that contains the vertex sets
in question. Even speedier wold be for me to at least say if there is
AT LEAST one link between the two vertex sets, although the total
number would be better to have. I have to do this fast and for many
times (many pairs of subgraphs).

Any help, ideas or simple encouragements would be kindly appreciated,
Sergiu