## Macro: SB-EXT:DEFINE-CAS-EXPANDER
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> Analogous to DEFINE-SETF-EXPANDER. Defines a CAS-expansion for ACCESSOR.
> BODY must return six values as specified in GET-CAS-EXPANSION.
> 
> Note that the system provides no automatic atomicity for CAS expansion, nor
> can it verify that they are atomic: it is up to the implementor of a CAS
> expansion to ensure its atomicity.
> 
> EXPERIMENTAL: Interface subject to change.

### Lambda list
```cl
(sb-impl::accessor sb-impl::lambda-list &body sb-impl::body)
```