code: def set_preferred_prefix_for_namespace(self, ns_uri, prefix, add_if_not_exist=False):
          
          ni = self.__lookup_uri(ns_uri)
  
          if not prefix:
              ni.preferred_prefix = None
          elif prefix in ni.prefixes:
              ni.preferred_prefix = prefix
          elif add_if_not_exist:
              self.add_prefix(ns_uri, prefix, set_as_preferred=True)
          else:
              raise PrefixNotFoundError(prefix)
docs: Sets the preferred prefix for ns_uri.  If add_if_not_exist is True,
          the prefix is added if it's not already registered.  Otherwise,
          setting an unknown prefix as preferred is an error.  The default
          is False.  Setting to None always works, and indicates a preference
          to use the namespace as a default.  The given namespace must already
          be in this set.
  
          Args:
              ns_uri (str): the namespace URI whose prefix is to be set
              prefix (str): the preferred prefix to set
              add_if_not_exist (bool): Whether to add the prefix if it is not
                  already set as a prefix of ``ns_uri``.
  
          Raises:
              NamespaceNotFoundError: If namespace ``ns_uri`` isn't in this set.
              DuplicatePrefixError: If ``prefix`` already maps to a different
                  namespace.