code: public static String getRelativeURI( HttpServletRequest request, String uri, PageFlowController relativeTo )
      {
          String contextPath = request.getContextPath();
          if ( relativeTo != null ) contextPath += relativeTo.getModulePath();
          int overlap = uri.indexOf( contextPath + '/' );
          if ( overlap == -1 ) return null;
          return uri.substring( overlap + contextPath.length() );
      }
docs: Get a URI relative to the URI of the given PageFlowController.
  
  @param request the current HttpServletRequest.
  @param uri the URI which should be made relative.
  @param relativeTo a PageFlowController to which the returned URI should be relative, or
  <code>null</code> if the returned URI should be relative to the webapp root.