code: private Optional<Data<Input, Output>> match_(Input input) {
          return Optional.ofNullable(recent_.get())
                  .filter((Data<Input, Output> data) -> equality_.test(data.getInput(), input));
      }
docs: Try to match an input against the most recent calculation.
  @param input The most recent input.
  @return An optional Data object, if the input matches the remembered calculation.