def xcombinations(items, n):
     """
     >>> print list(xcombinations(list("abcd"), 3))
     [('a', 'b', 'c'), ('a', 'b', 'd'), ('a', 'c', 'd'), ('b', 'c', 'd')]
     >>> print len(list(xcombinations(range(14), 3))) == 14*13*12/6
     True
     >>> print len(list(xcombinations(range(14), 8))) == 14*13*12*11*10*9*8*7/4
0320
     True
     >>> print len(list(xcombinations(range(24), 8))) == 24*23*22*21*20*19*18*1
7/40320
     True
     >>> print list(xcombinations(list(""), 2))
     []
     >>> print list(xcombinations(list("1234"), 2))
     [('1', '2'), ('1', '3'), ('1', '4'), ('2', '3'), ('2', '4'), ('3', '4')]
     """
     if n==0:
         yield (())
     else:
         for i in xrange(len(items)):
             for tailcomb in xcombinations_inner(items[i + 1:], n-1):
                 yield tuple((items[i],) + tailcomb)

 def xcombinations_inner(items, n):
     if n==0:
         yield (())
     else:
         for i in xrange(len(items)):
             for tailcomb in xcombinations_inner(items[i + 1:], n-1):
                 yield tuple((items[i],) + tailcomb)

 if __name__ == "__main__":
     import doctest
     import cProfile
     cProfile.run('doctest.testmod()')
     #doctest.testmod()