+;;; Tags for the main low-level types are stored in the low three
+;;; bits to identify the type of a machine word. Certain constraints
+;;; apply:
+;;; * EVEN-FIXNUM-LOWTAG and ODD-FIXNUM-LOWTAG must be 0 and 4: code
+;;; which shifts left two places to convert raw integers to tagged
+;;; fixnums is ubiquitous.
+;;; * LIST-POINTER-LOWTAG + 4 = OTHER-POINTER-LOWTAG: NIL is both a
+;;; cons and a symbol (at the same address) and depends on this.
+;;; See the definition of SYMBOL in objdef.lisp
+;;; * OTHER-POINTER-LOWTAG > 4: Some code in the SPARC backend,
+;;; which uses bit 2 of the ALLOC register to indicate that
+;;; PSEUDO-ATOMIC is on, doesn't strip the low bits of reg_ALLOC
+;;; before ORing in OTHER-POINTER-LOWTAG within a PSEUDO-ATOMIC
+;;; section.
+;;; (These are just the ones we know about as of sbcl-0.7.1.22. There
+;;; might easily be more, since these values have stayed highly
+;;; constrained for more than a decade, an inviting target for
+;;; inventive abstraction-phobic maintainers.:-)