1 A non-mutable value was assigned a value.
3 Erroneous code example:
10 let ss = SolarSystem { earth: 3 };
11 ss.earth = 2; // error!
14 To fix this error, declare `ss` as mutable by using the `mut` keyword:
21 let mut ss = SolarSystem { earth: 3 }; // declaring `ss` as mutable