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Check Extensions have two distinct forms: one of them extends when giving check, the other - when evading it. In each case, typical depth to extend is one ply. The reason behind check extension is that we are in a forcing sequence, so that it is desirable to know its outcome with more certainty, and the number of replies to check is limited, so we do not have to be afraid of a search explosion. Also, not extending checks may easily lead to the horizon effect, delaying the threat so far that the program cannot see it. However, some programmers don't extend checks (and captures) with negative SEE or even reduce them. Robert Hyatt claimed a significant gain in Crafty by doing so [1] [2].

If a program does not consider checks in the quiescence search, then we should take care that it does not enter it while in check. This is also a form of check extension [3]. In its most straightforward form, check extension is implemented in TSCP.

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References

  1. ^ Re: move count based pruning by Robert Hyatt, CCC, September 04, 2010
  2. ^ search extensions by Robert Hyatt, CCC, November 08, 2014
  3. ^ checks in q-search by Robert Hyatt, CCC, September 02, 2008

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