def bool_2_indices(a):
    """
    Convert boolean array into a 2D array of (start, stop) pairs.
    """
    if any(a):
        lims = []
        lims.append(np.where(a[:-1] != a[1:])[0])

        if a[0]:
            lims.append([0])
        if a[-1]:
            lims.append([len(a) - 1])
        lims = np.concatenate(lims)
        lims.sort()

        return np.reshape(lims, (lims.size // 2, 2))
    else:
        return None