Common Lisp the Language, 2nd Edition

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19.7. Structures of Explicitly Specified Representational Type

Sometimes it is important to have explicit control over the representation of a structure. The :type option allows one to specify that a structure must be implemented in a particular way, using a list or a specific kind of vector, and to specify the exact allocation of structure slots to components of the representation. A structure may also be ``unnamed'' or ``named,'' according to whether the structure name is stored in (and thus recoverable from) the structure.