[igraph] New igraph feature - shrink graph

[Tom Gregorovic]

Hi,
I plan to implement function for igraph that would shrink graph
according to vertex membership to give a global point of view on the
network. The same feature is for example in Pajek.
I want to consult it before I start.
First thing I would need is function that will reindex membership
vector to have member IDs continuous starting from 0, to match
vertices in shrunken graph. Is this function somewhere in igraph
already implemented? Vertices with one member ID (-1 as default) set
via parameter should be considered not to be shrunken (will get unique
ID in reindex function).
Second thing is what method to use for constructing shrunken graph.
The simple way would be to create empty graph with vertices according
to number of unique member IDs and then add edge by edge. One question
is if it would add multiple edges.
The function should also return which vertices were shrunken to which
vertex (as vector of vector?) and similar thing for edges (in case of
merging multiple edges).
Thank you in advance for your feedback.

Regards,
Tom Gregorovic