Select moves according to their weights in the book. If false the move
with the highest weight is selected.
-=item B<BookRandom> (default: true)
+=item B<BookLearn> (default: false)
-Select moves according to their weights in the book. If false the move
-with the highest weight is selected.
+Store learning information in the book (which must be writable). Currently
+no engine actually uses this information.
=item B<BookDepth> (default: 256)