TextWriter& operator<<(TextWriter& txt, const PMatrix<T,N,C>& d)
XMLWriter& operator<<(XMLWriter& xml, const PMatrix<T,N,C>& d)
TextWriter& operator<<(TextWriter& txt, const PMatrix<T,N,C>& d){  for(int j=0; j < N; ++j)    for(int i=0; i < N; ++i)      txt << d.elem(i,j);  return txt;}