abstract class IFoo
{
  public abstract IFoo foo(IFoo a);
}

class IFoo_int : IFoo {
  private int content = 0;
  public IFoo_int(int content) {
    this.content = content;
  }
  
  public override IFoo foo(IFoo a)
  {
      return this;
  }
}
class MainClass
{
    static void Main(string[] args)
    {}
}