code: private static URI asRefBase( JsonString uri)
      {
      try
        {
        return
          uri == null
          ? null
          : new URI( uri.getChars().toString());
        }
      catch( Exception e)
        {
        throw new ProjectException( String.format( "Error defining reference base=%s", uri), e);
        }
      }
docs: Returns the reference base URI represented by the given JSON string.