Bug 87856 [Commands] Last closed editor leaked due to reference by command manager
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/Command.java
* This flag can be set to a particular command identifier if only that
	 * command should print information to <code>System.out</code> when
	 * changing handlers.
	 */
	public static String DEBUG_HANDLERS_COMMAND_ID = null;
	/**
	 * The category to which this command belongs. This value should not be
	 * <code>null</code> unless the command is undefined.
	 */
	private Category category = null;
	/**
	 * A collection of objects listening to changes to this command. This
	 * collection is <code>null</code> if there are no listeners.
	 */
	private Collection commandListeners = null;
	/**
	 * The handler currently associated with this command. This value may be
	 * <code>null</code> if there is no handler currently.
	 */
	private IHandler handler = null;
	/**
	 * The ordered array of parameters understood by this command. This value
	 * may be <code>null</code> if there are no parameters, or if the command
	 * is undefined. It may also be empty.
	 */
	private IParameter[] parameters = null;
	/**
	 * Constructs a new instance of <code>Command</code> based on the given
	 * identifier. When a command is first constructed, it is undefined.
	 * Commands should only be constructed by the <code>CommandManager</code>