private FunctionBasedConverter(
        Function<? super A, ? extends B> forwardFunction,
        Function<? super B, ? extends A> backwardFunction) {
      this.forwardFunction = checkNotNull(forwardFunction);
      this.backwardFunction = checkNotNull(backwardFunction);
    }

    @Override
    protected B doForward(A a) {
      return forwardFunction.apply(a);
    }

    @Override
    protected A doBackward(B b) {

    private FunctionBasedConverter(
        Function<? super A, ? extends B> forwardFunction,
        Function<? super B, ? extends A> backwardFunction) {
      this.forwardFunction = checkNotNull(forwardFunction);
      this.backwardFunction = checkNotNull(backwardFunction);
    }

    @Override
    protected B doForward(A a) {
      return forwardFunction.apply(a);
    }

    @Override
    protected A doBackward(B b) {

// that.
template <typename Gradient>
class ForwardFunction
    : public std::function<Status(const std::vector<Gradient*>&,
                                  std::vector<Gradient*>*, bool)> {
 public:
  template <typename lambda_type>
  explicit ForwardFunction(lambda_type lambda)
      : std::function<Status(const std::vector<Gradient*>&,