Changes

Futility pruning is not used when the [[Side to move|side to move]] is in [[Check|check]] , or when either [[Alpha|alpha]] or [[Beta|beta]] are close to the [[Checkmate#MateScore|mate value]], since it would leave the program blind to certain [[Checkmate|checkmates]]. [[Tord Romstad]] reported that in his early program [[Gothmog]] one more condition was necessary - namely that futility pruning requires checking for the existence of at least one legal move <ref>[http://www.open-aurec.com/wbforum/viewtopic.php?f=18&t=43669&p=166791 Serious bug in Gothmog 0.2.6!] by [[Tord Romstad]], [[Computer Chess Forums|Winboard Forum]], August 04, 2003</ref> <ref>[https://www.stmintz.com/ccc/index.php?id=339076 Re: Unexpected problem with futility pruning?] by [[Tord Romstad]], [[CCC]], December 29, 2003</ref> to avoid returning erroneous stalemate scores. As replied by [[Eli David|Omid David]]: then simply return alpha (to fail [[Fail-Low|low]] but [[Fail-Hard|hard]]).