-;;; the main types. These types are represented by the low three bits
-;;; of the pointer or immediate object.
+;;; 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