The Hero King piece and chess variant, is it possible to implement in Fairy-Stockfish?

[EskoRanta]

A royal king piece that that moves like standard chess king but cannot be checked (let alone captured) by any opposing color piece from "front", that is, from a rank which is higher than the king itself from the kings viewpoint, is a Hero King. The Hero King is an asymmetrical piece by its shielding.

Replacing the wimpy standard chess kings with the Hero Kings changes the tactics and strategies of the game drastically because the Hero King is a fearsome front line warrior, but on the other, still quite vulnerable when surrounded alone by enemy pieces.

My dream is to get to see strong engines play the Hero King variant. I am sure it would look intriguing and entertaining for anybody who is a decent standard chess player.

Would the Hero King piece and therefore the Hero King Chess variant be possible to define in current Fairy-Stockfish?

This far I have been able to modify Douglas Bagnall's P4wn 2.01 Javascript program to play the Hero King Chess variant, at a mediocre level. If somebody wants to try it, the source can be found in a dedicated Facebook group (I'm sorry) https://www.facebook.com/groups/1004645991298592

[ianfab]

This unfortunately can not be configured in Fairy-SF. Making checks on the king conditional generally is very problematic implementation-wise. So if you want to try to implement it yourself, my recommendation would be to first try a slight variation where the king is not royal and has to be captured to win, because then the only thing you need to implement is to restrict captures of the king/commoner in Position::legal with your conditions, and other than that it could just work like other extinction variants.