Strange class precedence list in sbcl

In sbcl,

*(sb-mop:class-precedence-list (find-class 'cons))
==>(#<BUILT-IN-CLASS CONS> #<BUILT-IN-CLASS LIST> #<BUILT-IN-CLASS SEQUENCE>
 #<BUILT-IN-CLASS T>)

Isn't it stran开发者_如何学Goge that cons inherits from list and not the other way around? What am I missing here?

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This is per the specification. A LIST is either a CONS, or a symbol NIL (which is the only object of type NULL), which means that both of those types are specializations of LIST, and hence their equivalent system classes inherit from LIST.

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Not all conses are lists (since the ultimate cdr might not be nil), and not all lists are conses (nil is not a list, as mentioned earlier). So technically speaking, neither is of a class that is properly a subclass of the other. I think the spec is written that way because someone saw a practical use, but I agree that it's confusing and it might be misguided.