-;;; Array access functions return an object from the array, hence its
-;;; type is going to be the array upgraded element type.
+;;; Array access functions return an object from the array, hence its type is
+;;; going to be the array upgraded element type. Secondary return value is the
+;;; known supertype of the upgraded-array-element-type, if if the exact
+;;; U-A-E-T is not known. (If it is NIL, the primary return value is as good
+;;; as it gets.)