In the lists labs and your previous assignments, you've been exposed to HOF blocks like map, keep, and combine. You likely used them to carry out relatively simple tasks, like summing a list with combine, or filtering based on word length with keep. In this lab, you're going to attack some more complicated problems with the help of HOFs. In addition, you're going to see how functions can be treated as data, and how you can define your own blocks that take in functions as input, just like map, keep, and combine do.