code: @SuppressWarnings("unchecked")
      public static <S, I, O> ExtensionResult<S, I, O> empty() {
          return (ExtensionResult<S, I, O>) EMPTY;
      }
docs: Utility method, returning the (singleton) object indicating, no result could be computed.
  
  @param <S>
  (hypothesis) state type
  @param <I>
  input alphabet type
  @param <O>
  output alphabet type
  
  @return the empty result