Combinatorial numbering rank and unrank

Translate between standard numbering and combinatorial numbering of n choose k.

The sequence of combinations defined by N-choose-k (no ordering, no replacement) form a sequence that can be lengthy. These routines provide a way to translate between this sequence and the standard sequence of whole numbers and to advance within the sequence. I find this quite useful when one desires to break this sequence into equal chunks for parallel processing purposes. These sets are assumed to be sorted into lexicographic (like alphabetic) order.
Short example with n=5, k=3:
set,s rank,r
------ -------
[1 2 3] 0
[1 2 4] 1
[1 2 5] 2
[1 3 4] 3
[1 3 5] 4
[1 4 5] 5
[2 3 4] 6
[2 3 5] 7
[2 4 5] 8
[3 4 5] 9
r = kSubsetLexRank(s,k,n)
s = kSubsetLexUnrank(r,k,n)
s = kSubsetLexSuccessor(curset,k,n)