Added necessary "at" in line 170

[DreadedHippy]

I had to read this line more than 4 times to understand it. Having the "at" makes it more readable

[chriskrycho]

I can definitely sympathize with that experience, so I am sorry you had it! Unfortunately, the change actually makes the sentence incorrect grammatically, and the original is correct grammatically. We might be able to tweak it so the sentence reads like this, though:

Note that a reference’s scope starts from where it is introduced and continues through the last time that reference is used. For instance, this code will compile because the last usage of the immutable references, in the println!, occurs before the mutable reference is introduced:

That is still grammatically correct, preserving the structure of the sentence. Would it have helped with your confusion here?