1. ABAQUS elsets are not unique.. elements can belong to multiple elsets. However as far as I can see,
libmesh elements can belong to only one subdomain.. being able to be part of multiple subdomains would be very useful though..
2. MeshBase only allows a unique subdomain name - subdomain id mapping.. and giving subdomains ids depending on both elset name and element type makes this very hard to use the subdomain_name() function. I do not understand the original motivation for the implementation and I think a simpler scheme should not pose too many problems.
I have implemented code to enable the reader to handle generated element and node sets (start, end , step) as well as surfaces that are defined through element sets.