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.