When compiling for value. See Appendix B . See preceding footnote on future pluperfect. Via means we admittedly haven't explained yet. I did not make this one up this time.
Somebody can probably get a thesis out of that.
The value tag for a constant is itself. Again, this flows from the definition of constancy and the unsetqability and imperishability of constants. Experiment and consultation have shown that indeed this alternative works. Bear in mind that it is not the concept of idups, but marking them specially, which is open to question. It is possible to setq a variable without using the AQ, if you are wondering about that.