[akasakahakada]

The mechanism is simply called recursion. Or tail recursion specifically.

[hacksoi]

Tail recursion is completely irrelevant to this algorithm.

[akasakahakada]

then enlighten me how different is it from tail recursion?

[hacksoi]

Tail recursion is when the recursive call is last in a function; this algorithm uses breadth-first search to simultaneously 1. search the space of inputs and outputs 2. find the smallest set out of outputs to reach a reward. Again, tail recursion is completely irrelevant.