def prior_rev(C, alpha=-1.0):
    r"""Prior counts for sampling of reversible transition
    matrices.

    Prior is defined as

    b_ij= alpha if i<=j
    b_ij=0         else

    The reversible prior adds -1 to the upper triagular part of
    the given count matrix. This prior respects the fact that
    for a reversible transition matrix the degrees of freedom
    correspond essentially to the upper, respectively the lower
    triangular part of the matrix.

    Parameters
    ----------
    C : (M, M) ndarray or scipy.sparse matrix
        Count matrix
    alpha : float (optional)
        Value of prior counts

    Returns
    -------
    B : (M, M) ndarray
        Matrix of prior counts

    """
    ind = np.triu_indices(C.shape[0])
    B = np.zeros(C.shape)
    B[ind] = alpha
    return B