Summary of functions/definitions expected to return a number:           

  r>n       r<0       Source
--------- ----------  ------------------------------
error      error      MS Excel PERMUT() and COMBIN() 
error      error      HP32SII nPr and nCr    
undefined  undefined  CRC Combinatoric definitions   
undefined  undefined  Handbook of Mathematics and Computational Science  
undefined  undefined  Wolfram:  
http://mathworld.wolfram.com/Permutation.html
zero       zero       http://en.wikipedia.org/wiki/Choose_function  
undefined  undefined  http://en.wikipedia.org/wiki/Combination   
undefined  undefined  http://en.wikipedia.org/wiki/Permutation   
zero       undefined  Knuth's choose function in Concrete Mathematics    
zero       zero       GAP's nrCombinations()


Summary for tools that return the individual subsets:           

  r>n       r<0       Source
--------- ----------  ------------------------------
emptylist  error      Mathematica
emptylist  unknown    Magma
emptylist  error      GAP Combinations
emptylist  emptylist  GAP Arrangements
emptylist  emptylist  Sage Combinations
emptylist  error      Sage Permutations