+ * The sorter operates on fastbufs containing sequences of items. Each items
+ * consists of a key, optionally followed by data. The keys are represented
+ * by fixed-size structures of type SORT_KEY internally, if this format differs
+ * from the on-disk format, explicit reading and writing routines can be provided.
+ * The data are always copied verbatim, unless the sorter is in the merging
+ * mode in which it calls callbacks for merging of items with equal keys.
+ *
+ * All callbacks must be thread-safe.