Some combinatorics?

[dsimcha]

== Quote from bearophile (bearophileHUGS at lycos.com)'s article
> I'd like to write combinations, permutations, etc ranges (generators), for
std.algorithm or for a small std.combinatorics Phobos module. (I have written this
code time ago for dlibs1 in D1).
> I have found some small performance problems in converting the opApply
combinatorics code to the Range protocol.
> Are you interested in this module (or in just those few generators for
std.algorithm)? I find them often useful.
> Bye,
> bearophile

I've included these in my dstats lib, but my implementations admittedly aren't
very well thought out.  (I wrote these in 2008 before ranges and shoehorned them
into ranges.  I never fixed them because I kept thinking I or someone else was
going to rewrite them and that hasn't happened so far.)  I need this often enough
that I'd be interested in having them in Phobos.

Some code by Philippe Sigaud does it better than my implementations
(http://svn.dsource.org/projects/dranges/trunk/dranges/docs/algorithm.html) and is
Boost licensed.  He was thinking of getting parts of this lib into Phobos, but I
don't know what the status of that is.  If this project is abandoned and you like
it, you could take it over, or you could just write your own.

One important issue is whether the number of elements for a combination should be
specified at compile time (more efficient), runtime (more flexible) or both
(possibly bloated).

As far as where these should go, I'd say either std.range or a module publicly
imported by std.range.  They deal with range topology.