Bug 275929 [CommonNavigator] IPipelinedTreeContentProvider.getPipelinedElements() not ever called
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentServiceContentProvider.java
INavigatorContentDescriptor aDescriptor, Set theEnabledExtensions) {
		if (aDescriptor.getSuppressedExtensionId() != null /*
															 * The descriptor is
															 * an override
															 * descriptor
															 */
				&& aDescriptor.getOverridePolicy() == OverridePolicy.InvokeAlwaysRegardlessOfSuppressedExt) {
			/*
			 * if the policy is set as such, it can lead to this extension being
			 * invoked twice; once as a first class extension, and once an
			 * overriding extension.
			 */
			if (theEnabledExtensions.contains(contentService
					.getExtension(aDescriptor.getOverriddenDescriptor()))) {
				return true;
			}
		}
		return false;
	}
	
	/**
	 * Currently this method only checks one level deep. If the suppressed
	 * extension of the given descriptor is contained lower in the tree, then
	 * the extension could still be invoked twice.
	 * 
	 * @param aDescriptor
	 *            The descriptor which may be overriding other extensions.
	 * @param theEnabledDescriptors
	 *            The other available descriptors.
	 * @return True if the results should be pipelined through the downstream