  In this article we review standard null-move pruning and introduce our
extended version of it, which we call verified null-move pruning. In verified
null-move pruning, whenever the shallow null-move search indicates a fail-high,
instead of cutting off the search from the current node, the search is
continued with reduced depth.
  Our experiments with verified null-move pruning show that on average, it
constructs a smaller search tree with greater tactical strength in comparison
to standard null-move pruning. Moreover, unlike standard null-move pruning,
which fails badly in zugzwang positions, verified null-move pruning manages to
detect most zugzwangs and in such cases conducts a re-search to obtain the
correct result. In addition, verified null-move pruning is very easy to
implement, and any standard null-move pruning program can use verified
null-move pruning by modifying only a few lines of code.
