-If you compare `&i32` to `&'a i32`, they're the same, it's just that the
-lifetime `'a` has snuck in between the `&` and the `i32`. We read `&i32` as "a
-reference to an i32" and `&'a i32` as "a reference to an i32 with the lifetime 'a.'"
+If you compare `&mut i32` to `&'a mut i32`, they're the same, it's just that the
+lifetime `'a` has snuck in between the `&` and the `mut i32`. We read `&mut i32` as "a
+mutable reference to an i32" and `&'a mut i32` as "a mutable reference to an i32 with the lifetime 'a.'"