code: public void update() {
  		super.update();
  
  		if (!canModifyEditor()) {
  			setEnabled(false);
  			return;
  		}
  
  		ITextEditor editor= getTextEditor();
  		if (fOperationTarget == null && editor != null)
  			fOperationTarget= (ITextOperationTarget) editor.getAdapter(ITextOperationTarget.class);
  
  		boolean isEnabled= (fOperationTarget != null && fOperationTarget.canDoOperation(ITextOperationTarget.PREFIX) && fOperationTarget.canDoOperation(ITextOperationTarget.STRIP_PREFIX));
  		setEnabled(isEnabled);
  	}
docs: Implementation of the <code>IUpdate</code> prototype method discovers
  the operation through the current editor's
  <code>ITextOperationTarget</code> adapter, and sets the enabled state
  accordingly.