public class SumVisitor: Visitor{
  public int Visit (IntInterval iv){
    int res = 0;
    int lower = Math.Min(iv.From,iv.To),
        upper = Math.Max(iv.From,iv.To);

    for (int i = lower; i <= upper; i++) 
       res += i;
    return res;
  }

  public int Visit (IntSingular iv){
    return iv.TheInt;
  }

  public int Visit (IntCompSeq iv){
    return (iv.First.Accept(this) + 
            iv.Second.Accept(this));
  }
}