[BioC] Rkeys function from AnnotationDbi returns all Rkeys for a subset

[Laurent Gautier]

Inversion of "edge" and "vertex" in parts of my previous email.

Some people will have unconsciously corrected it. The others will be 
very confused.

Here is what it should read:

Here the subset operation takes a subset of the "mapping", that is of
the edges in the bipartite graph, without eliminating the unconnected
vertices. I suppose that this choice can be defended by the fact that 
vertices
in an AnnDbBimap object can be without any associated edge, which is
making sense. For example, in the context of microarray some probes can
be on the array, no given association be associated with it, but yet it
is practical to have such probes ID defined in a part (left or right) of
the BiMap.