select the slice.
@c ??? How does this work?
+ The most convenient way is to set the optimal slice by using
+BoundingBox information automatically determined from the document by
+typing @kbd{s b} (@code{doc-view-set-slice-using-mouse}).
+
@findex doc-view-reset-slice
To cancel the selected slice, type @kbd{s r}
(@code{doc-view-reset-slice}). Then DocView shows the entire page