Understanding `symbol??` macro from okmij.org

[Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>]

Hello Guile Users!

I am having trouble understanding how the `symbol??` macro from 
https://okmij.org/ftp/Scheme/assert-syntax-rule.txt works.

Here is what I have so far:

~~~~start~~~~
;; (symbol?? FORM KT KF)
;; FORM is an arbitrary form or datum.
;; A symbol? predicate at the macro-expand time.
;; Expands in KT if FORM is a symbol (identifier), Otherwise, expands in KF.
;; KT: Continuation/Case in case of true.
;; KF: Continuation/Case in case of false.
(define-syntax symbol??
   (syntax-rules ()
     ;; The check is done by first pattern matching against some other
     ;; forms, that are not a symbol.
     ((symbol?? (x . y) kt kf) kf) ; It's a pair, not a symbol
     ((symbol?? #(x ...) kt kf) kf) ; It's a vector, not a symbol
     ;; After those things are excluded, the thing might be a symbol.
     ((symbol?? maybe-symbol kt kf)
      (let-syntax ((test
                    (syntax-rules ()
                      ((test maybe-symbol t f) t)
                      ((test x t f) f))))
        (test abracadabra kt kf)))))
~~~~~end~~~~~

So I don't understand why `test` works or how it works to check, whether 
`maybe-symbol` is a symbol or not. And I don't understand how `abracadabra` 
helps with that, or why it helps, to pass something undefined.

Can someone help me understand, what is going on there?

Actually I am trying to understand the whole page, and I got there while looking 
for a good `assert` macro ... rabbit hole and all that.

Best regards,
Zelphir

-- 
repositories:https://notabug.org/ZelphirKaltstahl