[igraph] how to "simplify" a graph

[Sam Steingold]

(no, I am not asking about the simplify function)
I have a directed graph with, say, 100 sources and 100,000 destinations,
and only 5 sources which are also destinations.
I am mostly interested in sources, but the destinations clutter
everything.
I want to "simplify" the graph by dropping all the destinations, but I
want to add a "weighted" link between two sources which have common
destinations.
How do I do that?
thanks!
-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
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