=item B<BookLearn> (default: false)
-This is a noop.
+Switches on updating of the 'learn fields' of the opening book,
+which for each move record the number of times the move was played,
+and the number of half-points scored with it.
+It also switches on making use of this information already in the book,
+for altering the move-selection probabilities from those
+specified by the book weights.
+(Rarely played moves will be favored,
+moves with a significant poor performance will be shunned.)
+This option must be set before the book is specified,
+or the updating of the learn info will not work!
=item B<BookDepth> (default: 256)