Give even more bonus to passed pawn if adjacent squares to its path
are not attacked.
passed STC
http://tests.stockfishchess.org/tests/view/
5d08c9b10ebc5925cf0a6630
LLR: 2.95 (-2.94,2.94) [0.50,4.50]
Total: 175197 W: 39859 L: 38816 D: 96522
passed LTC
http://tests.stockfishchess.org/tests/view/
5d0ab8240ebc5925cf0a8fe4
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 92928 W: 16124 L: 15682 D: 61122
Bench:
3398333