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...t (and they have constant versions in the current implementation). Old workspaces will never have this bit set, so nothing will be re-created as constants (except NULL). If a workspace with some of these constant flag bits set is read by an old version of R, the flag bits will just be ignored (by UnpackFlags in serialize.c), so the objects will be restored the same as if they had been written by such an old version. So this should all work fine unless R Core implementations start using this bit for something else. (Or unless some old version of R used it for something else - which isn't the case...