Challenge: Proper burgers

// Here's the explanation of the challenge -- when you're done reading it,
// click on `main.js` above to get started with the challenge!

// Here at Nirvana Burgers, our burgers are very simple, but delicious --
// and yet, our new chef keeps messing them up somehow! We don't have time
// for another metaphysical kitchen debate on what constitutes a proper
// burger, so we'll have to settle the matter with code.

// ## Description

// In this challenge, you will write a `isProperBurger` function that receives
// a list (array) as an argument. Each element of the list is a string
// containing the name of an ingredient. Together, the list represents the
// whole stack of ingredients in the burger, ordered from bottom to top --
// in the order you'd put them on the plate.

// For example, for a simple cheese burger, your function could be invoked
// like:

// isProperBurger(
//   ["bread", "patty", "cheese", "bread"])

// Your function should return a boolean (`true` or `false`) that represents
// whether the burger is properly made. Here's all you need to know about the
// art of making proper burgers:

// - A burger always starts and ends with `"bread"`. There should not be
// bread anywhere else in the burger.
// - There has to be **at least one** `"patty"` **in between** the two pieces
// of bread. If there's no patty, then it's not a burger, it's just a pile of
// bread.
// There cannot be `"cheese"` under the patties. Cheese always goes on top
// (that is, on the list, it must appear _after_ the patties)

// Our burgers really are simple -- you can be assured that every element in
// the list of ingredients will be one of the ingredients mentioned above.