Changes

It has become practice to use forward-pruning methods only in the [[Principal Variation Search#ZWS|NWS]] part of the PVS framework. The idea is that it is too risky to prune forward at an expected [[Node Types#PV|PV-node]], because a possible mistake causes an immediate change of the [[Principal variationVariation|principal variation]].

If one erroneously forward prunes at some [[Node Types#ALL|All-node]] and the resulting score is backed up all the way to a PV-node, it obtain a [[Fail-High|fail-high]] and a re-search will be performed. The algorithm is then searching for a new principal variation and regards the subsequent nodes as PV-nodes. Because no forward pruning is done at PV-nodes, it is possible to find out whether there exists a new [[Principal variationVariation|PV]] or not. In this case the result of the mistake will be that an extra amount of nodes has been searched. In principle, forward pruning at an expected All-node is not dangerous but can trigger unnecessary re-searches.