[igraph] average.path.length

[Thomas]

If g looks like this:

http://www.nottingham.ac.uk/~liztc/M30603Probs.png

can anyone explain what average.path.length(g) will calculate? I  
assume it will ignore the nodes that are unconnected, but how would it  
deal with paths between a node in one group and a node in another  
group not connected to the first?

Thank you,

Thomas