code: public String lookupNamespace(String prefix)
      {
          String uri = null;
          final Stack stack = getPrefixStack(prefix);
          if (stack != null && !stack.isEmpty()) {
              uri = ((MappingRecord) stack.peek()).m_uri;
          }
          if (uri == null)
              uri = EMPTYSTRING;
          return uri;
      }
docs: Use a namespace prefix to lookup a namespace URI.
  
  @param prefix String the prefix of the namespace
  @return the URI corresponding to the prefix, returns ""
  if there is no visible mapping.