-Later:
-Let choice of time control, allow correspondance play, several games at the same time
-==> need to use indexedDB instead of localStorage. Maybe with Dexie https://dexie.org/
-Each user would have a unique identifier stored in the client DB.
-Allow to cancel games (if opponent doesn't connect again)
-Identity would be browser-based: different games on smartphone, home computer, work computer... (why not ?)
-Index might still look the same, and variant page would have another tab "Games"
-==> running, and finished (which can be deleted from local memory)
-(A true analysis mode could be implemented also, to navigate in completed games --> use a button)
-Allow challenging a specific player (by his chosen name)
-But keep the random pairings as main playing way + always playing in ZEN mode,
-except when accepting an individual challenge.
+Merge Orda + Empire + Hoppelpoppel + Newzealand getSlideNJumpMoves() into base_rules.js
+=> allow to simplify getPawnMoves in RoyalRace (and some getXMoves in Shatranj)