+;;; something which can be decoded into the inline expansion of the
+;;; function, or NIL if there is none
+;;;
+;;; To inline a function, we want a lambda expression, e.g.
+;;; '(LAMBDA (X) (+ X 1)). That can be encoded here in one of two
+;;; ways.
+;;; * The value in INFO can be the lambda expression itself, e.g.
+;;; (SETF (INFO :FUNCTION :INLINE-EXPANSION-DESIGNATOR 'FOO)
+;;; '(LAMBDA (X) (+ X 1)))
+;;; This is the ordinary way, the natural way of representing e.g.
+;;; (DECLAIM (INLINE FOO))
+;;; (DEFUN FOO (X) (+ X 1))
+;;; * The value in INFO can be a closure which returns the lambda
+;;; expression, e.g.
+;;; (SETF (INFO :FUNCTION :INLINE-EXPANSION-DESIGNATOR 'BAR-LEFT-CHILD)
+;;; (LAMBDA ()
+;;; '(LAMBDA (BAR) (BAR-REF BAR 3))))
+;;; This twisty way of storing values is supported in order to
+;;; allow structure slot accessors, and perhaps later other
+;;; stereotyped functions, to be represented compactly.