code: def updatePrefix(self, p, u):
          
          if p in self.nsprefixes:
              self.nsprefixes[p] = u
          for c in self.children:
              c.updatePrefix(p, u)
          return self
docs: Update (redefine) a prefix mapping for the branch.
          @param p: A prefix.
          @type p: basestring
          @param u: A namespace URI.
          @type u: basestring
          @return: self
          @rtype: L{Element}
          @note: This method traverses down the entire branch!