protected override void OnModelPropertyChanged( string propertyName )
        {
            if( propertyName == "Presentation" )
            {
                if( Presentation == Model.Presentation )
                {
                    return;
                }

                Presentation = Model.Presentation;

                Labels.Clear();

                var converter = new GenericLabelConverter( ConversionSteps );
                var captionModule = Presentation.GetPropertySetFor<Caption>();
                foreach( var node in Presentation.Graph.Nodes )
                {
                    var label = new LabelViewModel( captionModule.Get( node.Id ).Label );
                    label.Commited = converter.Convert( label.Original );

                    Labels.Add( label );
                }

                OnPropertyChanged( "Labels" );
            }
        }