/// <summary>
 /// Adds the specified VisualArc to the visual presentation of the petrinet.
 /// </summary>
 /// <param name="arc">A reference to the VisualArc that is to be added.</param>
 public void AddArc(IVisualArc arc)
 {
     VisualArcs.Add(arc.Id, arc);
 }
 /// <summary>
 /// Removes the specified VisualArc from the visual presentation of the petrinet.
 /// </summary>
 /// <param name="arc">The VisualArc to remove.</param>
 public void RemoveArc(IVisualArc arc)
 {
     VisualArcs.Remove(arc.Id);
 }