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>  * A tool can be in the following states: <br>
>  * disabled<->enabled[unusable<->usable[active<->inactive]]
>  * <->always_usable[active<->inactive]<->disabled
>  * <br> where each square bracket indicates a state nesting level
>  * and arrows possible state transitions.
>  * Unusable tools are always inactive as well and disabled tools
>  * are always unusable as well.
>  * State changes are propagated to registered ToolListeners.
>  *
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> 	 * An active tool is the currently selected tool in the
> 	 * DrawingView. A tool can be activated/deactivated
> 	 * by calling the activate()/deactivate() method.
> 	 *
> 	 * @return true if the tool is the selected tool in the DrawingView, false otherwise
> 	 * @see #isEnabled
> 	 * @see #isUsable
> 	 */
> 	public boolean isActive();
> 
> 	/**
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> 	/**
> 	 * A tool must be enabled in order to use it and to activate/deactivate it.
> 	 * Typically, the program enables or disables a tool.
> 	 *
> 	 * @see #isUsable
> 	 * @see #isActive
> 	 */
> 	public boolean isEnabled();
> 	public void setEnabled(boolean enableUsableCheck);
> 
> 	/**
> 	 * A usable tool is a enabled and either active or inactive.
> 	 * Typically, the tool should be able to determine itself whether it is
> 	 * usable or not.
> 	 *
> 	 * @see #isEnabled
> 	 * @see #isUsable
> 	 */
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> 	public void setUsable(boolean newIsUsable);