@Override
  public boolean equals(Object obj)
  { if(obj instanceof List)
    { List that=(List)obj;
      if(this.next==null && that.next==null)
      { return this.a.equals(that.a);}
      else if(this.next!=null && that.next!=null)
      { return this.a.equals(that.a)&&this.next.equals(that.next);}
      else
      { return false;}    
    }  
    else { return false;}
  }