+;;; * 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.:-)