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Changes made by the module properties dialog to project files are not undoable. For example, modifying the list of public packages changes the project.xml file, but undoing that modification requires either changing the properties back (if one remembers), hand-editing project.xml, or reverting to the last CVS version.
Sorry, this is a much more general issue than apisupport: the IDE does not support Undo for any file operations. When we have local file history, that could perhaps be used for this purpose.