@Override
  public W<DeltaPT> delta(DC obj)
  { if(obj instanceof PT)
    { PT that=(PT)obj;
      if(this.t.equals(that.t))
      { return new W(new Equality(true), new Similarity(0.0),new DeltaPT.Id(this));
      }
      else
      { return new W(new Equality(false), new Similarity(1.0),new DeltaPT(this,that));
      }
    }
    else
    { throw new RuntimeException(obj+" is NOT type of PT.");
    }
  }