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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def filter(self, mask):
 """ Create a SiteCollection with only a subset of sites. :param mask: Numpy array of boolean values of the same length as the site collection. ``True`` values should indicate that site with that index should be included into the filtered collection. :returns: A new :class:`SiteCollection` instance, unless all the values in ``mask`` are ``True``, in which case this site collection is returned, or if all the values in ``mask`` are ``False``, in which case method returns ``None``. New collection has data of only those sites that were marked for inclusion in the mask. """

        assert len(mask) == len(self), (len(mask), len(self))
        if mask.all():
            # all sites satisfy the filter, return
            # this collection unchanged
            return self
        if not mask.any():
            # no sites pass the filter, return None
            return None
        # extract indices of Trues from the mask
        indices, = mask.nonzero()
        return self.filtered(indices)