the full symmetric matrix
 [  1   2   3   4   5  ]
 [  2   6   7   8   9  ]
 [  3   7  10  11  12  ]
 [  4   8  11  13  14  ]
 [  5   9  12  14  15  ]

 the symmetric matrix in packed storage
 [  1  ]
 [  2   6  ]
 [  3   7  10  ]
 [  4   8  11  13  ]
 [  5   9  12  14  15  ]
 

 typedef matrix < double,
                  symmetric, 
                  packed<>,
                  row_major >::type SymmMatrix;
 

  double da[16];
  typedef matrix< double,
                  symmetric<lower>, 
                  packed<external>,
                  column_major >::type Matrix;
  const Matrix::size_type matrix_size = 5;
  Matrix A(da, matrix_size, matrix_size);
  typedef dense1D<double> Vec;
  Vec y(matrix_size,1),x(matrix_size), Ax(matrix_size);
  double alpha=1, beta=0;
  //          1   2   3   4   5        1        1000
  //          2   6   7   8   9        2        2000
  //     A =  3   7  10  11  12    x = 3    y = 3000
  //          4   8  11  13  14        4        4000
  //          5   9  12  14  15        5        5000

  //make A
  for (i = 0; i < 15; ++i)
    da[i] = i + 1;
  //make x y
  for ( i = 0; i < matrix_size; ++i){
    y[i]=1000*(i+1);
    x[i]=i+1;
  }
  mult(A, scaled(x,alpha), scaled(y,beta), y);

More examples can be found in symm_banded_vec_prod.cc