Using built-in abstract functions is an important skill to master for the rest of the course. Many other languages have started to include mechanisms like this, so you will find it useful going forward. As always, it helps to think in terms of the information consumed and produced, and the data that represents that information. How does the input data need to be transformed to get the result? Does that makes what one of the built in abstract functions do? Could it be a composition?
It helps to use the example step of the recipe to work out exactly what needs to happen. Remember, the harder the problem the more time you should spend on examples.
The solution for this problem set is: