Empty opening book * Opening book file format: * - 1 byte: board width * - 1 byte: board height * - 1 byte: max stored position depth * - 1 byte: key size in bits * - 1 byte: value size in bits * - 1 byte: log_size = log2(size). number of stored elements (size) is smallest prime number above 2^(log_size) * - size key elements * - size value elements */