Up to this point, we have considered variables to be either mutable or immutable, and that's it. However, this is not the case.

Consider the following:

let mut a = 10; 
let b = &mut a; 

What does this actually mean? First, we create a mutable binding to a that initially contains the value 10.

Next, we create an immutable binding to b that contains a reference to the mutable value of a.