A permutation f is said to be fixedpointfree,
provided there is
no x with f(x) = x.
The number of fixedpointfree permutations of an nelement set
is denoted by !n (called "subfactorial" of n),
Example: For n = 3, there are 6 permutations,
the two yellow ones are fixedpointfree:

 3 1 2  
 2 3 1 

(the fixed points are marked in red)