public void Remove(ExpressionElement element)
 {
     elements.Remove(element);
 }
 public void Update(int index, ExpressionElement element)
 {
     elements[index] = element;
 }
 public void Insert(int index, ExpressionElement element)
 {
     elements.Insert(index, element);
 }
 public void Add(ExpressionElement element)
 {
     elements.Add(element);
 }