This is the latest scan of errors and oddities in regexps in the Emacs source tree.
New this time is an experimental check for branch subsumption: whether one branch in an or-expression matches a superset of another, like "[ab]\\|a". Please tell me if you believe this might be useful, so that I know whether to include it in the next release of xr.

The algorithm uses some simple linear heuristics since a full regexp subset check would be quite expensive and probably require DFA construction and graph equivalence; maybe something for a future version.