@Override
  public boolean equals(Object obj)
  { if(obj instanceof Element)
    { Element that=(Element)obj;
      if(this.i!=that.i||
         this.j!=that.j||
         !this.a.equals(that.a)||
         !this.b.equals(that.b)
         )
      { return false;
      }
      else
      { return true;
      }
    }
    else
    { throw new RuntimeException(obj+" is not an Element.");
    }
  }