code: public String lookupPrefix(String uri)
      {
          String foundPrefix = null;
          Enumeration prefixes = m_namespaces.keys();
          while (prefixes.hasMoreElements())
          {
              String prefix = (String) prefixes.nextElement();
              String uri2 = lookupNamespace(prefix);
              if (uri2 != null && uri2.equals(uri))
              {
                  foundPrefix = prefix;
                  break;
              }
          }
          return foundPrefix;
      }
docs: Given a namespace uri, and the namespaces mappings for the
  current element, return the current prefix for that uri.
  
  @param uri the namespace URI to be search for
  @return an existing prefix that maps to the given URI, null if no prefix
  maps to the given namespace URI.