Complete conclusions for cycles chapter

book/src/pages/cycles/conclusions.md

@@ -2,12 +2,20 @@
 
 This chapter has focused on function composition.
 We've used `Angle => Point` functions to represent parametric curves,
-and we have seen that we can build many useful abstractions around them by using function composition.
+and we have seen that we can build many useful abstractions based on function composition.
 
 Functions are not the only things that compose. 
 Every time we've used the `beside`, or `above` methods on `Image`, for example, we've been composing images.
-This notion of composition is one of the key ideas in functional programming, and we'll continue to see it throughout the book.
+Composition is one of the key ideas in functional programming, and we'll continue to see it throughout the book.
+These methods are sometimes known as **combinators**.
+This isn't a term we'll use much, but you may find it in other literature so it's useful to know what it means.
 
-closure
+The methods of composition, or combinators, all have a property known as **closure**.
+This means that the input is the same sort of thing as the output.
+When we combine two `Images` using `above` we get back an `Image`.
+When we combine two functions using `andThen` we get back a function.
 
-combinators
+It isn't necessary for all the parameters of a method to have the same type for a method to be a combinator.
+For example, `strokeColor` combiines an `Image` and a `Color` to produce an `Image`. 
+We still consider this a combinator because we're most interested in the `Image` type, which is both an input and an output of `strokeColor`.
+So we could say that `strokeColor` composes an `Image` and a `Color` to produce an `Image`.