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question about subtypep

On p. 35 of CLtL, it says that the type "array" may or may not be a
subtype of "common", depending on whether your implementation allows you
to make arrays that can only hold items of a type which isn't a subtype
of common.  Supposing the implementation has not made this extension, is
it acceptable for (subtypep 'array 'common) to return T?  Or is subtypep
only supposed to return T if the subtype relationship definitely holds
in *all* CL implementations?