lbls, np.mean, float, 0)
    array([ 2.75,  5.5 ,  6.  ])

    Falling back to `default`:

    >>> lbls = np.arange(1, nlbl+2)
    >>> ndimage.labeled_comprehension(a, lbl, lbls, np.mean, float, -1)
    array([ 2.75,  5.5 ,  6.  , -1.  ])

    Passing positions:

    >>> def fn(val, pos):
    ...     print("fn says: %s : %s" % (val, pos))
    ...     return (val.sum()) if (pos.sum() % 2 == 0) else (-val.sum())
    ...
    >>> ndimage.labeled_comprehension(a, lbl, lbls, fn, float, 0, True)
    fn says: [1 2 5 3] : [0 1 4 5]
    fn says: [4 7] : [ 7 11]
    fn says: [9 3] : [12 13]
    array([ 11.,  11., -12.,   0.])

    Nz