+;;; Check to see whether OBJECT is a circular reference, and return something
+;;; non-NIL if it is. If ASSIGN is T, then the number to use in the #n= and
+;;; #n# noise is assigned at this time. Note: CHECK-FOR-CIRCULARITY must
+;;; be called *EXACTLY* once with ASSIGN T, or the circularity detection noise
+;;; will get confused about when to use #n= and when to use #n#. If this
+;;; returns non-NIL when ASSIGN is T, then you must call HANDLE-CIRCULARITY
+;;; on it. If you are not using this inside a WITH-CIRCULARITY-DETECTION,
+;;; then you have to be prepared to handle a return value of :INITIATE which
+;;; means it needs to initiate the circularity detection noise. See the
+;;; source for info on how to do that.