Do exercise 21.1 on page 468.
Do exercise 21.2 on page 468.
Do exercise 21.5 on page 469.
Exercise 23.x1: Using the natural semantis for Language One, show how the conclusion
times(const(3),plus(const(2),const(1))) → 9
would be derived. That is, what are the immediately preceding premises from which a rule allows this conclusion to be derived? And for any of these immediately preceding premises that is itself a consequence of applying some rule to earlier premises, what are those?
Exercise 23.x3: Using the natural semantics for Language Two, show how the conclusion
< let(x,const(3),times(var(x),const(5))), [ ] > → 15