How to make `set!` change the variable in `let` (Scheme)?
Recently when I self-learnt MIT 6.5151 course, I have read SICP 1 to 2.1 as ps0 requires (also read 2.2.1 as CS 61A notes requires) and then Software Design for Flexibility (SDF) Prologue, chapter 1 and partly Appendix on Scheme.