Appendix: list functions

The ride builds most of its answers out of lists. These are the main List functions the chapters use, in the order they first come up.

Function	What it does	Chapter
`map`	Calls a function on each item, and returns a list of the answers	Lists and map: a far tree
`map_with_index`	`map`, with each item's place in the list, counting from 0	Lists and map: a far tree
`concat`	Puts a second list after the first	Lists and map: a far tree
`join`	Makes one list of a list of lists	The pipeline
`keep_oks`	Calls a function that returns a `Try` on each item, keeps the `Ok` values, and drops the `Err`s	Errors as tag unions
`List.from_iter`	Collects a count, such as `U32.until(0, n)`, into a list	The road
`join_map`	Maps each item to a list, and joins the lists into one	Rails
`drop_first`	Drops the first items, as many as it's given	Rails
`fold`	Hands each item in turn to a function, along with the answer so far, starting from a first answer	The throttle
`find_first`	Returns the first item that passes a test, as a `Try`	The truck's motion
`List.repeat`	Makes a list of one value, as many times as it's given	The cat
`take_first`	Keeps the first items, as many as it's given	The math layer
`map2`	Walks two lists side by side, and calls a function on each pair	The math layer