How to extend a function?

["Joerg F. Wittenberger" <Joerg.Wittenberger@xxxxxxxxx>]

Hello,

I'd like to reuse some definitions from the docbook style sheets, that
is I want to define something for the same name, which shall special
case parts of the original and call the original one for those cases
which are still the same.  I could not find a way how to achive that.

Assumed the style sheet had something like

(define (xyz a)
 (cond
  ((equal? a "X") #f)
  (else #t)))

Now I want to "use" that style sheet and say

(define (xyz a)
 (cond
  ((equal? a "Y") #f)
  (else (xzy a))))

The last call should go to the first definition.  Possible?

Thanks

/Jerry


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