Implementing a custom :set function for an alist of key-directory pairs

I made the following customisable variable consisting of an alist 
of key-directory pairs.

The user can change the directory for each key at will. How does 
one construct the :set function exactly for this case?

Although I have looked at the Emacs lisp directory for :type 'alist, 
I could not find any :set function implementations.

(defcustom orellana-fpln
  (let ( (waypt  orellana-waypt) ) 
    `( (ORLNA . ,waypt)
       (GLXKS . ,(concat waypt "/galaxiakos"))
       (HKDMR . ,(concat waypt "/hakudo-maru"))
       (TKYNS . ,(concat waypt "/tachyons"))
       (SVMTR . ,(concat waypt "/servomotor"))
       (AMZNS . ,(concat waypt "/amazones"))
       (GRVDR . ,(concat waypt "/groovador"))
       (KGIZA . ,(concat waypt "/groovador/kasrgiza"))
       (MTJRA . ,(concat waypt "/montejura")) ))

  orellana-fpln-dokette

  :group 'orellana

  :type '(alist :key-type (symbol :tag "Code")
                :value-type (string :tag "Path"))

  :initialize 'custom-initialize-set

  :set #'orellana-fpln-custom-set)





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