[racket] clarification for beginners please

    Probably a crab would be filled with a sense of personal outrage
    if it could hear us class it without ado or apology as a crustacean,
    and thus dispose of it.  "I am no such thing, it would say: I am
    MYSELF, MYSELF alone."

                  - William James, The Varieties of Religious Experience


eq? is a function for letting us distinguish different crabs that would
otherwise look the same to us.


Let's say that we have two crabs c1 and c2.


;;;;;;;
;; In BSL:
;;;;;;;
(define-struct crab (name))

(define c1 (make-crab "joe"))
(define c2 (make-crab "joe"))
;;;;;;;


As it stands, we may want to say they are the same because they "look" the
same --- they have the same structural shape.  equal? tells us that they
look the same.

;;;;;;
> (equal? c1 c2)
true
;;;;;;


But as William James notes, these crabs might be outraged at our lack of
crustacean discrimination.  They may argue: we may look alike, but we are
different crabs.  How shall we judge?

;;;;;;
> (eq? c1 c2)
false
;;;;;;


A crab is itself, itself alone:

;;;;;;
> (eq? c1 c1)
true
;;;;;;
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