[plt-scheme] set-car! in EOPL language

Dear All,

On this bit of code:

(cases node (car nodelist)
                 (a-node (n fl neighs v)
                         (set-car! nodelist (a-node n fl node-neighbors v))))

where nodelist is a list of nodes (a defined datatype), I get this error:

Welcome to DrScheme, version 4.2 [3m].
Language: Essentials of Programming Languages (3rd ed.); memory limit:
128 megabytes.
. . reference to undefined identifier: set-car!
>

Why is set-car! undefined? Solution?
-- 

Cheers,

Marco