+2006-11-22: Updated OCamlMakefile.
+2006-07-26: Improved documentation.
+2004-07-01: Fixed is_empty-function. It now uses the newly available
+2004-04-19: Added split-function to ptset.
+2003-11-23: Updated OCamlMakefile.
+2003-01-07: Updated OCamlMakefile to make use of "findlib".
+2002-12-03: Replaced union1 and union2 with one union-function only.
+ Replaced inter1 and inter2 with one inter-function only.
+2002-09-25: Fixed example: it had mistakenly been changed during some
+ test application so that it did not do what it was supposed
+2002-09-11: Updated OCamlMakefile and license.
+ Documented all modules for use with ocamldoc.
+2002-05-04: Revised the whole installation procedure. See INSTALL for
+2002-04-30: Updated OcamlMakefile: it does not ask for confirmation
+ during installation anymore!
+2002-04-04: Renamed pretty-printing functions for consistency.
+2002-03-08: Added new functions to pomap (see interface documentation
+ * fold_split_eq_classes
+2002-02-27: Added new functions to pomap (see interface documentation
+ Added new functions to store:
+ Slightly modified the Hasse-example.
+2002-02-18: Added new functions to pomap (see interface documentation
+ * rev_topo_fold, rev_topo_foldi
+ * rev_chain_fold, rev_chain_foldi
+ Slightly extended the Hasse-example.
+2002-02-04: Fixed bug in function "is_empty".
+ Added functions "chain_fold" and "chain_foldi", which fold
+ over chains of ascending elements (see interface for details).
+2001-08-01: Fixed tiny bug in example "hasse".
+2001-07-31: Changed order of arguments for a few functions to make them
+ more similar to set/map-functions in the standard
+ library. Sorry for the inconvenience!
+2001-07-30: Added more modules to "pomap": they contain functionality
+ to generate specifications for the dot-utility to render
+ Hasse-diagrams. This functionality is now factored out of
+ the Hasse-example. See the following files for documentation:
+ The Hasse-example now demonstrates this functionality.
+2001-07-27: Incompatible change: "get_graph" was removed, "get_nodes"
+ introduced (with a slightly different type).
+2001-07-26: Added function "create_node".
+2001-07-26: Tiny optimization.
+2001-07-24: Incompatible change: the "Added"-constructor returned by
+ "add_find" now also tells you the added node. This was
+ important, because it could be interesting to know the
+ neighbours of a newly inserted node.
+2001-07-18: Added functions "topo_fold" and "topo_foldi" that fold
+ over elements in topological order.
+2001-07-17: First release.