ostream& operator<<(ostream& s, const PVector<T,N,C>& d)
XMLWriter& operator<<(XMLWriter& xml, const PVector<T,N,C>& d)
ostream& operator<<(ostream& s, const PVector<T,N,C>& d){  for(int i=0; i < N; ++i)    s << d.elem(i);  return s;}