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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 is_valid(self, tol: float = DISTANCE_TOLERANCE) -> bool: """ True if SiteCollection does not contain atoms that are too close together. Note that the distance definition is based on type of SiteCollection. Cartesian distances are used for non-periodic Molecules, while PBC is taken into account for periodic structures. Args: tol (float):
 Distance tolerance. Default is 0.5A. Returns: (bool) True if SiteCollection does not contain atoms that are too close together. """

        if len(self.sites) == 1:
            return True
        all_dists = self.distance_matrix[np.triu_indices(len(self), 1)]
        return bool(np.min(all_dists) > tol)