Set::IntSpan manages sets of integers. It is optimized for sets that
have long runs of consecutive integers. These arise, for example, in
.newsrc files, which maintain lists of articles:

alt.foo: 1-21,28,31
alt.bar: 1-14192,14194,14196-14221

Sets are stored internally in a run-length coded form. This provides
for both compact storage and efficient computation. In particular,
set operations can be performed directly on the encoded
representation.