def index_to_slices(index):
    """
    take a numpy array of integers (index) and return a  nested list of slices such that the slices describe the start, stop points for each integer in the index.

    e.g.
    >>> index = np.asarray([0,0,0,1,1,1,2,2,2])
    returns
    >>> [[slice(0,3,None)],[slice(3,6,None)],[slice(6,9,None)]]

    or, a more complicated example
    >>> index = np.asarray([0,0,1,1,0,2,2,2,1,1])
    returns
    >>> [[slice(0,2,None),slice(4,5,None)],[slice(2,4,None),slice(8,10,None)],[slice(5,8,None)]]
    """
    if len(index)==0:
        return[]

    #contruct the return structure
    ind = np.asarray(index,dtype=np.int)
    ret = [[] for i in range(ind.max()+1)]

    #find the switchpoints
    ind_ = np.hstack((ind,ind[0]+ind[-1]+1))
    switchpoints = np.nonzero(ind_ - np.roll(ind_,+1))[0]

    [ret[ind_i].append(slice(*indexes_i)) for ind_i,indexes_i in zip(ind[switchpoints[:-1]],zip(switchpoints,switchpoints[1:]))]
    return ret