}
}
+// Compute the sign of a permutation of a vector of things.
+template <typename T>
+int permutation_sign(vector<T> s)
+{
+ if (s.size() < 2)
+ return 0;
+ int sigma = 1;
+ for (typename vector<T>::iterator i=s.begin(); i!=s.end()-1; ++i) {
+ for (typename vector<T>::iterator j=i+1; j!=s.end(); ++j) {
+ if (*i == *j)
+ return 0;
+ if (*i > *j) {
+ iter_swap(i,j);
+ sigma = -sigma;
+ }
+ }
+ }
+ return sigma;
+}
+
// Collection of `construct on first use' wrappers for safely avoiding
// internal object replication without running into the `static
// initialization order fiasco'. This chest of numbers helps speed up