Hi all,

Attached is a patch that adds multiset functionality atop GSL 1.13.
Multisets are like combinations but with replacement-- a single item
may appear multiple times.  Element order is insignificant and
enumeration uses lexicographical ordering.

The patch builds very, very, very heavily on the existing combinations
code.  Documentation, an example, and unit tests are included.
Please consider it for inclusion in the next feature release.

- Rhys

P.S. One weakness in the patch the amount of code duplication relative
to combinations.  The underlying data structures are the same, and
only the _init_first, _init_last, _valid, _next, and _prev functions
differ in a non-trivial way.  With some thought to naming, multisets
could be a combinations-with-replacement flag.  If you accept the
patch, please consider adding a TODO to both directories to merge the
functionality.