Bug 125420 [Commands] parameters: allow commands to specify return type
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/Command.java
if (handler == null)
			return false;
		return handler.isHandled();
	}
	/**
	 * Removes a listener from this command.
	 * 
	 * @param commandListener
	 *            The listener to be removed; must not be <code>null</code>.
	 * 
	 */
	public final void removeCommandListener(
			final ICommandListener commandListener) {
		if (commandListener == null) {
			throw new NullPointerException(
					"Cannot remove a null command listener"); 
		}
		removeListenerObject(commandListener);
	}
	/**
	 * Removes a listener from this command.
	 * 
	 * @param executionListener
	 *            The listener to be removed; must not be <code>null</code>.
	 * 
	 */
	public final void removeExecutionListener(
			final IExecutionListener executionListener) {
		if (executionListener == null) {
			throw new NullPointerException(