🚀 go-pugleaf

There can be a slight improvement made to the zero window searching
that is carried out in gnuchess.  There are 2 places where a zero window
search is done.  The first is when NULLMOVE is defined and a null move
search is carried out.  Here a window of (beta, beta+1) is being used.
It turns out that we can use a window of (beta, beta).  Now if a score
of beta+1 is obtained, we have a beta cut whereas in the former case, this
will not give a cut.  This 1 point change makes a difference because
in the hashtable, a lowerbound is stored as beta+1.  What this means is
that when a lowerbound entry of beta+1 is found in the hashtable, a cut is
obtained whereas in the former, it would not be identified as a cut.

Similarly, during zero window searching in PVS, instead of using a 
window of (alpha, alpha+1), a window of (alpha, alpha) can be used.

Several experimental runs shows an improvement of 2-3% in the number of
nodes searched.


Kong Sian