'Roots of the graph of urns marked for deletion.'

@property
def root_urns_for_deletion(self):

    roots = set()
    for urn in self._urns_for_deletion:
        new_root = True
        str_urn = utils.SmartUnicode(urn)
        fake_roots = []
        for root in roots:
            str_root = utils.SmartUnicode(root)
            if str_urn.startswith(str_root):
                new_root = False
                break
            elif str_root.startswith(str_urn):
                fake_roots.append(root)
        if new_root:
            roots -= set(fake_roots)
            roots.add(urn)
    return roots
