code: @Override
      public void onPreAdd(final IoFilterChain chain, final String name,
              final NextFilter nextFilter) {
          if (chain.contains(ProxyFilter.class)) {
              throw new IllegalStateException(
                      "A filter chain cannot contain more than one ProxyFilter.");
          }
      }
docs: Called before the filter is added into the filter chain.
  Checks if chain already holds an {@link ProxyFilter} instance.
  
  @param chain the filter chain
  @param name the name assigned to this filter
  @param nextFilter the next filter
  @throws IllegalStateException if chain already contains an instance of
  {@link ProxyFilter}