I propose to repeat that #f is returned by the call to gardian and to
suggest that false is not a valid representative. See below in CAPITAL.

(guardian obj [ rep ])

Registers obj with guardian, specifying rep as the representative
object. If rep is omitted, the representative object is obj.
NOTE THAT #f SHOULD NOT BE USED AS REPRESENTATIVE.

(guardian)

Returns the representative of a single object OF THE INACCESSIBLE GROUP
from guardian and removes the object. WHEN NO OBJECT IS BELONGING IN
THE INACCESSIBLE GROUP, #f IS RETURNED.