Changes

a technique to extend along the [[Principal variationVariation|principal variation]] (PV) to achieve higher [[Depth|search depths]] in the critical part of the [[Search Tree|tree]]. In [[Rebel]], [[Ed Schroder|Ed Schröder]] incremented a counter each time two consecutive [[PV-Move|PV-moves]] were retrieved from the [[Transposition Table|transposition table]], to extend by one [[Ply|ply]] each time the counter is divisible by four <ref>[http://www.top-5000.nl/authors/rebel/chess840.htm#PVS Extension Techniques in REBEL (PVS extensions)] from [http://www.top-5000.nl/authors/rebel/chess840.htm Programmer Corner] by [[Ed Schroder|Ed Schröder]]</ref>. <span id="Multiple"></span>Influenced by observing the principal variations of [[Junior]] during the [[Kasparov versus Deep Junior 2003]] match, [[David Levy]] proposed multiple extensions to treat the often early stable part of a PV as a single ply in 2003 <ref>[[David Levy]] ('''2003'''). ''The State of the Art in Man vs. “Machine” Chess''. [[ICGA Journal]], Vol. 26, No. 1</ref>. To determine the length of the PV, Levy expresses a threshold as a percentage of the total number of nodes searched with the common PV during the previous iteration of the [[Iterative Deepening|iterative deepening]].