; In this section, Jay talks about big programs and how to get a
; step by step feeling for their functioning and structure without
; getting overly frightened by them.
; "I want you to get a feeling for their shape."
;
; What is a rule? (another form of substitution and abstraction)

; A Rule is a pattern, from which one generates a skeleton.
; Translated it means to match the expression source and out
; of that, instantialize an expression target.

; "Instead of bringing the rules to the level of the computer [...],
;  we're going to bring the computer to the level of us."

; 4A:(13m44s) Writing a general simplifier.
(DEFINE DSIMP
  (SIMPLIFIER DERIV-RULES))
; usage:
;   >> (dsimp '(dd (+ x y) x))
;    | (+ 1 0)

; An organic process.
;   * You stick an expression into a stew of enzymes or
;     bacteria and it changes. After a while you retract it.
;   * The enzymes can attach to any part of the expression.
; A parallel process.

; important QUESTIONS 4A:(21m38s) BREAK
;   How it works.

; 4A:(23m59s) => 4A:(35m34s) Get a feeling for big programs.
; "I just want you to look at the shape of it!" --Jay
; MATCH
;  * A box that takes as its input:
;      an expression
;      a pattern
;      a dictionary
;  * It then outputs another dictionary,
;      which is the result of augmenting the input-dictionary
;         with which was found in the expression and the pattern.
; 4A:(25m33s) The problem is, that the matcher has to compare
;   the expression with the pattern.

; 4A:(36m2s) => 4A:(36m46s) Get a feeling for big programs.
; INSTANTIATE
;  * takes:
;      a dictionary
;      a skeleton
;  * outputs:
;      an expression
; 4A:(35m52s) This is "easy".

; 4A:(38m48s) => 4A:(39m41s)
; "I don't want you to look at this in detail.
;  I want you to say, there is more going on here." --Jay
; EVALUATE
;  * takes: a form of magic from Lisp
; 4A:(38m48s)

; Do not think about concrete things!
; important QUESTIONS 4A:(40m14s) BREAK

; Garbage in, Garbage out Simplifier

; 4A:(50m27s) => (END) Get a feeling for big programs.
; "The key to very good programming, is to know what to not think about."
; "And whishful thinking." --Jay
;    In the end you only need specifications of what a thing does.
;    Just like in mathematics and philosophy.
;  ==> GIGO

; QUESTIONS 4A:(1h1m10s) (END)