**Exhibit
      4(b)**




EXECUTION
      VERSION







FIRST
      SUPPLEMENTAL INDENTURE




FIRST
      SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of
      October [__], 2007, to the Indenture dated as of July 1, 2003 (the
“Indenture”), by and between TXU Corp., a Texas corporation (the
“Company”) and The Bank of New York, a New York banking corporation,
      as
      trustee (the “Trustee”).




W
      I T N E S S E T H




WHEREAS,
      the Company and the Trustee have heretofore executed and delivered the Indenture
      providing for the issuance of an unlimited aggregate principal amount of
      unsecured debentures, notes or other indebtedness to be issued in one or more
      series;




WHEREAS,
      the Indenture provides that the terms of the securities may be set forth in
      an
      Officer’s Certificate;




WHEREAS,
      the Company has heretofore delivered to the Trustee an Officer’s Certificate,
      dated July 15, 2003 (the  “Series N Officer’s Certificate”),
      providing for the Floating Rate Convertible Senior Notes due 2033 (the
“Series N Notes”);




WHEREAS,
      the Company, Texas Energy Future Holdings Limited Partnership, a Delaware
      limited partnership (“Parent”), and Texas Energy Future Merger Sub Corp,
      a Texas corporation and a wholly owned subsidiary of Parent (“Merger
      Sub”), previously entered into the Agreement and Plan of Merger, dated as of
      February 25, 2007 (the “Merger Agreement”);




WHEREAS,
      Section 7(M) of the Series N Officer’s Certificate provides that upon a merger
      of the Company, as a result of which the holders of the common stock, without
      par value, of the Company (“Common Stock”) shall be entitled to receive
      stock, securities or other property or assets (including cash) with respect
      to
      or in exchange for such Common Stock, the Company or the successor or the
      purchasing corporation, as the case may be, shall execute with the Trustee
      a
      supplemental indenture providing that at the effective time of such merger,
      each
      Series N Note shall be convertible into the kind and amount of shares of stock
      and other securities or property or assets (including cash) receivable upon
      such
      merger by a holder of a number of shares of Common Stock issuable upon
      conversion of such Series N Note immediately prior to such merger;




WHEREAS,
      pursuant to the terms of the Merger Agreement, Merger Sub will be merged with
      and into the Company and the Company shall continue as the surviving entity
      (the
“Merger”), and pursuant to the Merger, each share of the Common Stock, no
      par value, of the Company issued and outstanding immediately prior to the
      effective time of the Merger, shall be converted into the right to receive
      $69.25 per share in cash;




WHEREAS,
      pursuant to Article 12 of the Indenture and Section 7(M) of the Series N
      Officer’s Certificate, the Trustee is authorized to execute and deliver this
      Supplemental Indenture;




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WHEREAS,
      the Company has been authorized by resolution to enter into this Supplemental
      Indenture;




WHEREAS,
      all other acts and proceedings required by law, by the Indenture and by the
      charter documents of the Company to make this Supplemental Indenture a valid
      and
      binding agreement for the purposes expressed herein, in accordance with its
      terms, have been duly done and performed; and




WHEREAS,
      the Company hereby requests that the Trustee execute and deliver this
      Supplemental Indenture;




NOW,
      THEREFORE, in consideration of the premises and the covenants and agreements
      contained herein, and for other good and valuable consideration the receipt
      of
      which is hereby acknowledged, the Company and the Trustee hereby agree as
      follows:




1.           AMENDMENT
      TO SERIES N OFFICER’S CERTIFICATE.




(a)
            Section
                7(A) of the Series N Officer’s Certificate, is hereby amended and restated
                to read in its entirety as follows:




“Right
      to Convert.  A
      Holder may convert its Series N Notes into an amount in cash specified below,
      during the periods and subject to the conditions stated in paragraph 5 of the
      Form of Series N Notes.  The amount of cash payable upon conversion of
      a Series N Note per $1,000 principal amount thereof shall be $4,274.05 (the
      “Conversion Rate”).  A Holder may convert a portion of the
      principal amount of Series N Notes if the portion is $1,000 or a multiple of
      $1,000.”




(b)
            Section
                7(B) of the Series N Officer’s Certificate is hereby further amended by
                amending and restating the third sentence thereof to read in its
                entirety
                as follows:




“Cash
      deliverable upon conversion will be delivered through the Conversion Agent
      as
      soon as practicable, but no later than the fifth Business Day following the
      Conversion Date.”




(c)
            Section
                7(B) of the Series N Officer’s Certificate is hereby further amended by
                deleting the fourth, fifth, sixth and eighth sentences thereof in
                their
                entirety.




(d)
            Section
                7(B) of the Series N Officer’s Certificate is hereby further amended by
                amending and restating the ninth sentence thereof to read in its
                entirety
                as follows:




“On
      conversion of Series N Notes, the
      portion of accrued interest including accrued Contingent Interest, if any,
      with
      respect to the converted Series




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N
      Notes shall not be cancelled, extinguished or forfeited, but rather shall be
      deemed to be paid in full upon delivery of the cash payment upon
      conversion.”




(e)
            Section
                7(B) of the Series N Officer’s Certificate is hereby further amended by
                amending and restating the tenth sentence thereof to read in its
                entirety
                as follows:




“Notwithstanding
      conversion of any
      Series N Notes, the Holders of the Series N Notes will continue to be entitled
      to receive Additional Amounts in accordance with the Registration Rights
      Agreement.”




(f)
            Section
                7(B) of the Series N Officer’s Certificate is hereby further amended by
                amending and restating the eleventh sentence to read in its entirety
                as
                follows:




“If
      a Holder converts more than one
      Series N Note at the same time, the amount of cash issuable upon the conversion
      shall be based on the total principal amount of the Series N Notes
      converted.”




2.           AMENDMENTS
      TO FORM OF SERIES N NOTES.




(a)
            Paragraph
                5 on the reverse of each Series N Note is hereby deemed to be amended
                by
                amending and restating the forepart of the first sentence of such
                Paragraph to read in its entirety as
                follows:




“Subject
      to the procedures set forth in
      the Indenture and the Officer’s Certificate, a Holder may convert Securities of
      this series into $4,274.05 in cash per $1,000 principal amount of Securities
      on
      or before the close of business on June 15, 2033 during the periods and upon
      satisfaction of at least one of the conditions set forth below:”




(b)
            Paragraph
                5 on the reverse of each Series N Note is hereby further deemed to
                be
                amended by amending and restating the third full paragraph to read
                in its
                entirety as follows:




“The
      amount of cash payable upon
      conversion of this Security (the “Conversion Rate”) per $1,000 principal amount
      hereof shall be $4,274.05.




3.           EFFECTIVENESS
      OF AMENDMENTS.  This Supplemental Indenture shall be effective at the
      Effective Time (as defined in the Merger Agreement).




4.           CAPITALIZED
      TERMS.  Capitalized terms used herein without definition shall have
      the meanings assigned to them in the Indenture or the Series N Officer’s
      Certificate, as the context shall require.




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5.           CONTINUING
      EFFECT OF INDENTURE.  Except as expressly provided herein, all of the
      terms, provisions and conditions of the Indenture, the Series N Notes and the
      Series N Officer’s Certificate shall remain in full force and
      effect.




6.           CONSTRUCTION
      OF SUPPLEMENTAL INDENTURE.  This Supplemental Indenture is executed as
      and shall constitute an indenture supplemental to the Indenture and shall be
      construed in connection with and as part of the Indenture.  THE LAWS
      OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED TO CONSTRUE THIS SUPPLEMENTAL
      INDENTURE.




7.           TRUST
      INDENTURE ACT CONTROLS.  If any provision of this Supplemental
      Indenture limits, qualifies or conflicts with another provision of this
      Supplemental Indenture or the Indenture that is required to be included by
      the
      Trust Indenture Act of 1939 as in force at the date as of which this
      Supplemental Indenture is executed, the provision required by said Act shall
      control.




8.           TRUSTEE
      DISCLAIMER.  The recitals and statements contained in this
      Supplemental Indenture shall be taken as the recitals and statements of the
      Company, and the Trustee assumes no responsibility for their
      correctness.  The Trustee makes no representations as to the validity
      or sufficiency of this Supplemental Indenture.  All rights,
      protections, privileges, indemnities and benefits granted or afforded to the
      Trustee under the Indenture shall be deemed incorporated herein by this
      reference and shall be deemed applicable to all actions taken, suffered or
      omitted by the Trustee under this Supplemental Indenture.




9.           COUNTERPARTS.  This
      Supplemental Indenture may be executed in any number of counterparts, each
      of
      which so executed shall be deemed to be an original, but all such counterparts
      shall together constitute but one and the same instrument.




10.           SUPPLEMENTAL
      INDENTURE FORMS PART OF INDENTURE.  This Supplemental Indenture is
      executed and shall be construed as an indenture supplemental to the Indenture
      and, as provided in the Indenture, this Supplemental Indenture forms a part
      of
      the Indenture for all purposes.  The Indenture, as amended and
      supplemented by this Supplemental Indenture, is in all respects ratified and
      confirmed.




11.           HEADINGS.  The
      section headings herein are for convenience only and shall not affect the
      construction hereof.




12.           SEVERABILITY.  In
      case any provision in this Supplemental Indenture, the Series N Officer’s
      Certificate or the Series N Notes shall be invalid, illegal or unenforceable,
      the validity, legality and enforceability of the remaining provisions shall
      not
      in any way be affected or impaired thereby.




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IN
      WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture
      to
      be duly executed as of the date first above written.







**TXU
                CORP.**By:Name:Title:












**THE
                BANK OF NEW YORK,**
              **as
                Trustee**By:Name:Title:


































*[Signature
      Page – Supplemental Indenture]*








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