• Matrix::value_type, VecX::value_type, and VecY::value_type must be the same type.
• The multiplication operator must be defined for Matrix::value_type.
• The addition operator must be defined for Matrix::value_type.

• A.nrows() == A.ncols()
• A.nrows() == x.size()
• x.size() == y.size()
• A has rectangle shape and is dense.

typedef matrix< double, 
                symmetric<lower>, 
                packed<>, 
                row_major>::type Matrix;
typedef dense1D<double> Vec;
  const int N = 3;
  const double alpha = 10;
  Matrix A(N,N);
  Vec y(N),x(N),ay(N),ax(N);
  //do the update
  rank_two_update(A, scaled(x, alpha), y);

  //print result
  cout << "axy^T + ayx^T + A:";
  print_all_matrix(A);