code: def to_matrix( xx, yy, zz, xy, yz, xz ):
      
      matrix = np.array( [[xx, xy, xz], [xy, yy, yz], [xz, yz, zz]] )
      return matrix
docs: Convert a list of matrix components to a symmetric 3x3 matrix.
      Inputs should be in the order xx, yy, zz, xy, yz, xz.
  
      Args:
          xx (float): xx component of the matrix.
          yy (float): yy component of the matrix.
          zz (float): zz component of the matrix.
          xy (float): xy component of the matrix.
          yz (float): yz component of the matrix.
          xz (float): xz component of the matrix.
  
      Returns:
          (np.array): The matrix, as a 3x3 numpy array.