code: def _purge_unreachable(self, key):
          
          dels = []
          for p in self:
              if dominates(key, p):
                  dels.append(p)
          for k in dels:
              _, idxs = find_in_tree(self.root, k, perfect=True)
              del_by_idx(self.root, idxs)
docs: Purge unreachable dominated key paths before inserting a new key
          path.