Check whether `s_indices` and `a_indices` are sorted in
    lexicographic order.

    Parameters
    ----------
    s_indices, a_indices : ndarray(ndim=1)

    Returns
    -------
    bool
        Whether `s_indices` and `a_indices` are sorted.

def _has_sorted_sa_indices(s_indices, a_indices):
    """
    Check whether `s_indices` and `a_indices` are sorted in
    lexicographic order.

    Parameters
    ----------
    s_indices, a_indices : ndarray(ndim=1)

    Returns
    -------
    bool
        Whether `s_indices` and `a_indices` are sorted.

    """
    L = len(s_indices)
    for i in range(L-1):
        if s_indices[i] > s_indices[i+1]:
            return False
        if s_indices[i] == s_indices[i+1]:
            if a_indices[i] >= a_indices[i+1]:
                return False
    return True