Author of R code: Benjamin Auder

C code (core algorithms) from a course chapter of Pierre Audibert:
  http://www.ai.univ-paris8.fr/~audibert/ens/07-ENUMERATIONx.pdf

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Get and print the 13th permutation of 1...5 :
  p = pinit(5)
  gonext(p, 12)
  show(p)

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Get and print the 7th combination of 1...5, p=2 :
  c = cinit(5, 2)
  gonext(c, 7)
  show(c)
