def _vec(A):
    """
    Linear operator _vec() from Wiktorsson2001 p478
    Args:
      A: a rank 3 array of shape N x m x n, giving a matrix A[j] for each
      interval of time j in 0..N-1
    Returns:
      array of shape N x mn x 1, made by stacking the columns of matrix A[j] on
      top of each other, for each j in 0..N-1
    """
    N, m, n = A.shape
    return A.reshape((N, m*n, 1), order='F')