[BioC] graph classes: how to combine multiple graphs into a single graph

[Paul Shannon]

help (package='graph')  describes many methods, but none of those mentioned seem to combine two graphs into a third -- perhaps even combining attributes along the way.

graph::union looked promising, but is defined such that it only works if the nodes in each graph are identical.

Any suggestions?  Perhaps objects of the new multigraph class can be constructed from multiple graphs?

Thanks -

 - Paul