add both incoming and previous

training-slides/src/shared-mutability.md

@@ -201,6 +201,7 @@ println!("{} {} {} {}", a, b, c, c2);
 ```
 
 The code works, but you *have* to use shadowing with `let a = &mut x.a;` or else the compiler will error. The borrowchecker is particularly frail here - replacing `Foo` with `x = [1,2,3]` and trying to borrow indexes will make it error out.
+Which does work but doesn't solve the problem of declaring `let mut x` to begin with.
 
 ## `RefCell`