def _agg_sum(iur, item, sims, use):
    """
    Sum aggregate

    Args:
        iur(matrix._CSR): the item-user ratings matrix
        item(int): the item index in ``iur``
        sims(numpy.ndarray): the similarities for the users who have rated ``item``
        use(numpy.ndarray): positions in sims and the rating row to actually use
    """
    x = 0.0
    for j in use:
        x += sims[j]
    return x