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A general description is here.
The new function ‘Rf_allocLang’ is now available. This provides an alternative to the idiom of calling ‘Rf_allocList’ followed by ‘SET_TYPEOF’.
‘SET_TYPEOF’ now signals an error unless the old and new types have compatible memory structure and content. Use of ‘SET_TYPE’ in package C code should be avoided and may be deprecated in the near future. It is better to allocate an object of the proper type in the first place.