Check to see whether any DataType we hold references to is included in the Stack (which holds all DataType instances that directly or indirectly reference this instance, including this instance itself).
The heart of the matter. This is where the selector gets to decide
on the inclusion of a file in a particular fileset. Most of the work for this selector is offloaded into SelectorUtils, a static class that provides the same services for both FilenameSelector and DirectoryScanner.
You can optionally reverse the selection of this selector, thereby emulating an <exclude> tag, by setting the attribute negate to true. This is identical to surrounding the selector with <not></not>.