Usage   : my @deranges = derange(@n);
 Function: implements !n, a derangement of n items in which none of the
           items appear in their originally ordered place.
 Example : my @n = qw(a b c);
           my @d = derange(@n);
           print join "\n", map { join " ", @$_ } @d;
           # prints:
           # a c b  ### Not a derangment; a in it's original position
           # b a c  ### Not a derangment; c in it's original position
           # b c a  
           # c a b 
           # c b a  ### Not a derangment; b in it's original position