code: @SuppressWarnings("unchecked")
      public <TO> ChainedTransformer<I, TO> chain(Transformer<O, TO> transformer) {
          if (transformer != null) {
              transformers.add(transformer);
          }
          return (ChainedTransformer<I, TO>) this;
      }
docs: Adds the specified transformer to the chain.
  
  @param transformer Transformer to be added.
  @param <TO>        Type of output of the specified transformer.
  @return This.