Circularity
If X is a man, X is a person
If X is person, X is human
If X is human, X is a person
This situation has been obtained by adding 'Carlo is a man' to the situation where there are only the three rules.
At this point, if the node 'Carlo is a man' becomes OUT, the two nodes 'Carlo is a person' and 'Carlo is human' would remain IN (they mutually support each other !!).
In this case well founded supports are utilized, i.e. supports which go back to premises.
Retractions are propagated only through well founded supports.