## Hints

The tests for this exercise require you to use extensions, a mechanism for adding new functionality to an existing class whose source you do not directly control without having to subclass it. To learn more about Kotlin's implementations of extensions, check out the [official documentation](https://kotlinlang.org/docs/reference/extensions.html#extensions).

The `customFoldLeft` and `customFoldRight` methods are "fold" functions, which is a concept well-known in the functional programming world, but less so in the object-oriented one. If you'd like more background information, check out this [fold](https://en.wikipedia.org/wiki/Fold_(higher-order_function)) page.