Enrich ifdef example.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13569 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Cleanup.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13570 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Doc.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13571 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Restore.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13572 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Doc.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13573 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02