code: def pset(self, n):
          
          s = set()
          for ns in n.nsprefixes.items():
              if self.permit(ns):
                  s.add(ns[1])
          return s
docs: Convert the nodes nsprefixes into a set.
          @param n: A node.
          @type n: L{Element}
          @return: A set of namespaces.
          @rtype: set