Algorithm code: Combination opposite to std::permutation

[BenBear]

Hi, libstdc++-v3

adjust_combination ()
next_combination ()
prev_combination ()

I wrote a algorithm of combination, which is opposite to
std::permutation.  It provides three operations, adjust_combination(),
next_combination() and prev_combination().  The next_combination() and
prev_combination() is just like the std::next_permutation() and
std::perv_permutation().

Each operation's arguments are three iterators, first, middle and last.
I treat the range [first, middle) as the elements selected, while the
range [middle, last) are the elements left.  The range [first, middle)
is treated in dictionary ordering, like the std::permutation.  [middle,
last) is maintained in a special ordering.

A simple example:

select 2 elements from {1, 2, 3, 4}
f   m   l              // f: first,   m: middle,    l: last
1 2, 3 4 
1 3, 4 2 
1 4, 3 2 
2 3, 4 1 
2 4, 3 1 
3 4, 2 1 

The first two number are the elements selected, in dictionary ordering.
The next combination of {3 4}/{2 1} will be {1 2}/{3 4}.

Ben Bear
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