This results to be a top concept, but it is unavoidable, since too many concepts can be interpretant of something else, thus making restrictions would be an unfortunate task! Moreover, it is ontologically meaningful to treat the something that interprets something else as a separate category and not as an abstract object; for example, the smoke behind the hill that can be an interpretant for the presence of persons is hardly conceivable as abstract.
(<=> (*Sign ?Self)
(And (Entity ?Self)
(Minimum-Slot-Cardinality ?Self Interpretant-Of 1)))