<PAGE>   1
                                                                  EXHIBIT 10.1.6

                               TENTH AMENDMENT TO
                        AGREEMENT OF LIMITED PARTNERSHIP
                      OF EOP OPERATING LIMITED PARTNERSHIP

     THIS TENTH AMENDMENT TO AGREEMENT OF LIMITED PARTNERSHIP OF EOP OPERATING
LIMITED PARTNERSHIP (this "AMENDMENT"), dated October 1, 1999, is entered into
by EQUITY OFFICE PROPERTIES TRUST, a Maryland real estate investment trust, as
managing general partner (the "GENERAL PARTNER") of EOP Operating Limited
Partnership, a Delaware limited partnership (the "PARTNERSHIP"), for itself and
on behalf of itself and the Limited Partners of the Partnership, and that or
those persons whose signatures are set forth on the signature pages attached
hereto.

     WHEREAS, on the date hereof, PALO ALTO SQUARE LIMITED PARTNERSHIP, an
Illinois limited partnership ("CONTRIBUTOR"), is receiving 1,012,623 Class B
Units of limited partnership interest in the Partnership ("OP UNITS") in
exchange for certain real property interests, pursuant to a closing under, and
as more particularly described in, that certain Contribution Agreement dated
September 28, 1999, by and between the General Partner (or its permitted
assignees) and Contributor (the "CONTRIBUTION AGREEMENT").  The Contributor is
the owner of certain property (the "PROPERTY") described in the Contribution
Agreement.

     WHEREAS, Contributor has assigned the OP Units to its constituent partners
as set forth on Schedule 1 who desire to become Limited Partners of the
Partnership (the "EQUITY HOLDERS").

     WHEREAS, pursuant to the authority granted to the General Partner under
the Agreement of Limited Partnership of the Partnership dated as of July 3,
1997, as amended thereafter (as so amended, the "PARTNERSHIP AGREEMENT"), the
General Partner desires to amend the Partnership Agreement to reflect (i) the
admission of the Contributor as an Additional Limited Partner, (ii) the
admission of the Equity Holders as Substituted Limited Partners and the holders
of the OP Units and (iii) certain other matters described herein.

     WHEREAS, the Contributor and the Equity Holders desire to become a party
to the Partnership Agreement and to be bound by all of the terms, conditions
and other provisions of this Amendment and the Partnership Agreement.

     NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration, the receipt and sufficiency of which hereby are
acknowledged, the General Partner hereby amends the Partnership Agreement as
follows:

     1.    ADMISSION TO PARTNERSHIP.  The Contributor is hereby admitted to the
Partnership as an Additional Limited Partner in accordance with Section 12.2 of
the Partnership Agreement and hereby agrees to become a party to the
Partnership Agreement and to be bound by all of the terms, conditions and other
provisions of the Partnership Agreement, including, but not limited to, the
power of attorney set forth in Section 15.11 of the Partnership Agreement. The
General Partner hereby consents to the transfer of the OP Units from the




<PAGE>   2
Contributor to the Equity Holders as reflected on Schedule 1 attached hereto.
Accordingly, the Equity Holders are hereby admitted to the Partnership as
Substituted Limited Partners in accordance with Section 11.4 of this
Partnership Agreement and hereby agree to become parties to the Partnership
Agreement and to be bound by all of the terms, conditions and other provisions
of the Partnership Agreement (as amended by this Amendment), including, but not
limited to, the power of attorney set forth in Section 15.11 of the Partnership
Agreement, and the Contributor is no longer a Limited Partner of the
Partnership.

     2.    RESTATEMENT OF EXHIBIT A.  Exhibit A to the Partnership Agreement
hereby is amended by replacing such Exhibit A with Exhibit "A" attached to this
Amendment.

     3.    RIGHT TO ASSIGN.  Notwithstanding any other provision of this
Amendment or of the Partnership Agreement, the Equity Holders shall have the
right to assign all or any portion of their OP Units, together with any and all
other rights of the Equity Holders pursuant to this Amendment or the
Partnership Agreement, to one or more of the constituent partners or
shareholders, members, partners or beneficiaries of constituent partners of the
Equity Holders on the date hereof, whether direct or indirect, without the need
for the consent of the Managing General Partner or any other General Partner or
Limited Partner and without being subject to the right of first refusal set
forth in Section 11.3.A(a) of the Partnership Agreement, but in each case
subject to the restrictions and conditions set forth in Sections 11.3.C,
11.3.D, 11.3.E, 11.6.E and 11.6.F of the Partnership Agreement.  Upon the
delivery of written notice of such an assignment to the Managing General
Partner, each assignee of OP Units pursuant to the immediately preceding
sentence shall be admitted to the Partnership as a Substituted Limited Partner
owning the OP Units so assigned and having all of the rights of a Limited
Partner under the Partnership Agreement and this Amendment, subject only to
such assignee executing and delivering, to the Partnership an acceptance of all
of the terms and conditions of the Partnership Agreement and such other
documents or instruments as the Managing General Partner may reasonably require
to effect such admission, in accordance with Section 11.4.B of the Partnership
Acreement.  Each permitted assignee of any of the OP Units, issued to the
Contributors pursuant to the Contribution Agreement and subsequently
transferred to the Equity Holders, that is admitted as a Substituted Limited
Partner in accordance with this Section 3 or Article XI of the Partnership
Agreement, for so long as such Person owns any such OP Units, is referred to in
this Amendment as an "Indirect Equity Holder."  Upon satisfaction of the
condition described in the second sentence of this Section 3, the Managing
General Partner shall amend Exhibit A to the Partnership Agreement in the manner
described in Section 11.4.C of the Partnership Agreement.  For purposes of
Section 8.6 of the Partnership Agreement, each Equity Holder and Indirect Equity
Holder shall be entitled to exercise its right to require the Partnership to
redeem all or any portion of the OP Units assigned to it by an Equity Holder at
any time on or after the date hereof.





                                       2
<PAGE>   3
     4.    ADJUSTMENTS TO CARRYING VALUES.

           (a)   The Carrying Values of the Assets of the Partnership shall be
adjusted in accordance with the procedures described in Section 1.D of Exhibit
B to the Partnership Agreement:  provided, however, that in order to minimize
the administrative burden associated with the adjustments required by this
Section 4(a) in connection with the distribution of the Cash Amount to an Equiy
Holder, or an Indirect Equity Holder, the Partnership shall make the
adjustments to the Carrying Values of the Partnership's Assets (and the
resulting adjustments to the Capital Accounts of the Partners) only upon the
happening of the most material event during the calendar year that is described
in Section 1.D(2) of Exhibit B to the Partnership Agreement (the "ANNUAL
ADJUSTMENT"); and provided further, that upon the distribution of the Cash
Amount to an Equity Holder or an Indirect Equity Holder or, at the option of
the General Partner, upon the occurrence of any other event described in
Section 1.D(2) of Exhibit B to the Partnership Agreement, that occurs during
any year other than as of the date of the Annual Adjustment, the Partnership
shall, at the time of such distribution, make adjustments to the Carrying
Values of the Partnership's Assets in accordance with the procedures described
in Section 1.D of Exhibit B to the Partnership Agreement for purposes of
adjusting the Capital Account of an Equity Holder, or such Indirect Equity
Holder who has exercised his Redemption Right or such other affected Partner,
but no such adjustments shall be necessary at such time with respect to the
Capital Account balances of Partners who remain Partners through the date of
the Annual Adjustment or are otherwise not directly affected by any such other
event.

           (b)   Any determination of the fair market value of Partnership
assets pursuant to Section 1.D of Exhibit B to the Partnership Agreement (for
purposes of calculating Unrealized Gain or Unrealized Loss) with respect to
adjusting the Carrying Values of Partnership assets in connection with the
exercise of Redemption Rights by an Equity Holder or any Indirect Equity Holder
shall be made by assuming that the aggregate fair market value of all
Partnership assets is equal to the aggregate Cash Amount that would be
distributed by the Partnership if all Partnership Units held by all Partners
(including the General Partners) were redeemed in exchange for the Cash Amount
with respect to each such Partnership Unit at such time, provided, however,
such valuation methodology shall not be utilized for purposes of determining the
fair market  value of the Partnership's assets with respect to any such exercise
of redemption Rights in contemplation of an assignment by or reorganization of
the Partnership for the benefit of creditors and any liquidation of the
Partnership related thereto or following the filing by (or in contemplation of a
filing) by the Partnership of a case under Title 11 of the U.S. Code.

     5.    ALLOCATIONS.  Nothwithstanding the provisions of Section 2.C of
Exhibit C to the Partnership Agreement, for purposes of allocating items of
income, gain, loss and deduction with respect to the Property in the manner
required by Section 704(c) of the Code, the Partnership shall employ, and shall
cause any entity controlled by the Partnership which holds title to any of the
Property to employ, the "traditional method" as set forth in Regulation Section
1.704-3(b).




                                       3
<PAGE>   4

     6.    OBLIGATION TO RESTORE DEFICIT CAPITAL ACCOUNTS.

           (a)   For purposes of this Section 6, the following terms shall have
the meanings set forth below:

                 (i)      "Aggregate Protected Amount" means the aggregate
                 balances of the Protected Amounts, if any, of all Protected
                 Partners, as determined on the date in question.

                 (ii)     "Protected Amount" means, with respect to any
                 Protected Partner, an amount equal to (i) the taxable gain, if
                 any, that would be realized by such Protected Partner if such
                 Partner were to dispose of its Partnership Interests for no
                 consideration other than the release or deemed release of
                 liabilities of the Partnership assumed by or otherwise
                 allocable to such Partner under Code Section 752, as such
                 hypothetical gain is determined from time to time, less (ii)
                 such Partner's share of "qualified nonrecourse financing" as
                 defined in Code Section 465(b)(6) and the Treasury Regulations
                 thereunder, as such share is determined from time to time in
                 accordance with Treasury Regulations Section 1.752-3(a).  The
                 Protected Amount allocable to any Protected Partner may be
                 modified from time to time by an amendment to the Agreement or
                 by execution of a written instrument by and between such
                 Protected Partner and the General Partner, acting on behalf of
                 the Partnership and without the prior written consent of any
                 other Partner (whether or not Protected Partners).

                 (iii)    "Protected Partner(s)" means that or those Limited
                 Partner(s) designated as Protected Partner(s) on Exhibit A
                 attached hereto and made a part hereof, as such designation
                 may be modified from time to time by the General Partner,
                 whether by express amendment to this Agreement or by execution
                 of a written instrument by and between any Protected
                 Partner(s) and the General Partner, acting on behalf of the
                 Partnership and without the prior consent of other Limited
                 Partners (whether or not Protected Partners). For purposes
                 hereof, any successor, assignee, or transferee of Partnership
                 Interests from a Protected Partner, which successor, assignee
                 or transferee determines its basis in such Units by reference
                 to the basis of the predecessor, assignor or transferor
                 Protected Partner, shall be considered a Protected Partner for
                 purposes hereof.

                 (iv)     "Recourse Liabilities" means the amount of
                 liabilities owed by the Partnership (other than Nonrecourse
                 Liabilities and liabilities to which Partner Nonrecourse
                 Deductions are attributable in accordance with Treasury
                 Regulations Section 1.704-2(i)).

           (b)   Notwithstanding any provision of the Partnership Agreement to
the contrary (including, without limitation, the second sentence of Section
13.3 of the Partnership



                                       4
<PAGE>   5
Agreement), upon liquidation of the Partnership or upon the liquidation of the
Partnership Interest of a Protected Partner, such Protected Partner whose
interest is being liquidated shall contribute to the Partnership in accordance
with Treasury Regulation Section 1.704-1(b)(2)(ii)(b)(2), the deficit balance,
if any, in its Capital Account, calculated after the allocation for such year of
all items of Net Income, Net Losses, Gross Income and Unrealized Gain or
Unrealized Loss allocated in accordance with Section 6.1 of the Partnership
Agreement, Section l.D of Exhibit B to the Partnership Agreement, and modified
by Section 7 of this Amendment; provided, however, that in no event shall such
contribution obligation for any Protected Partner exceed such Protected
Partner's Protected Amount.  The obligation created pursuant to this Section
6(b) shall be for the benefit of the Partnership, its general partners (the
"GENERAL PARTNERS"), the creditors of the Partnership or any other person to
whom any debts, liabilities or obligations are owed by (or who otherwise has any
claim against) the Partnership or the General Partners in their capacities as
general partners of the Partnership and shall be enforceable by such parties.
Each Protected Partner unconditionally and irrevocably waives any subrogation,
reimbursement or similar rights to which it might otherwise be entitled as the
result of its performance with respect to the obligation created pursuant to
this Section 6(b), whether such rights arise with respect to the Partnership,
another Partner of the Partnership or a third party; provided, however, that the
General Partners shall in all events be entitled to enforce the contribution
obligation of a Protected Partner undertaken pursuant to Section 6(b).
Notwithstanding anything herein to the contrary, the obligation of a Protected
Partner to contribute amounts pursuant to this paragraph shall continue in full
force and effect in accordance with the terms hereof until one year following
the transfer or other disposition by such Protected Partner of its Partnership
Interests unless the Partnership and such Protected Partner agree otherwise.
Unless expressly agreed by the parties to the contrary in this or an other
agreement, any Partner of the Partnership who is not a Protected Partner shall
have no obligation to contribute amounts to the Partnership.

     7.    ALLOCATIONS OF NET INCOME AND NET LOSSES.

           For purposes of maintaining Capital Accounts and in determining the
rights of the Partners among themselves, Partnership income, gain, loss and
deductions (computed in accordance with Exhibit B of the Partnership Agreement)
shall be allocated among the Partners in each taxable year (or portion thereof)
as provided in Section 6.1 of the Partnership Agreement, subject to the
following adjustments:

           (a)   Net Income.  After giving effect to the special allocations
set forth in Section 1 of Exhibit C of the Partnership Agreement, Net Income
otherwise allocable to the General Partners pursuant to the provisions of
Section 6.1.A(i) of the Partnership Agreement shall be allocated:

                 (i)      first, to the General Partners to the extent that
                 cumulative Net Losses previously allocated the General
                 Partners pursuant to Section 7(b)(iii) of this Amendment
                 exceed cumulative Net Income previously allocated to the
                 General Partners pursuant to this clause (i);




                                       5
<PAGE>   6

                 (ii)     next, to each Protected Partner until the cumulative
                 Net Income allocated such Protected Partner under this clause
                 (ii) equals the cumulative Net Losses allocated such Protected
                 Partner under Section 7(b)(ii) of this Amendment (and, within
                 the class of Protected Partners, pro rata in proportion to
                 their respective percentages of the cumulative Net Losses
                 allocated all Protected Partners pursuant to Section 7(b)(ii)
                 hereof); and

                 (iii)    thereafter, to the General Partners until the
                 cumulative Net Income allocated under this clause (iii) equals
                 the cumulative Net Losses allocated the General Partners under
                 Section 7(b)(i) of this Amendment.

         (b)     Net Losses.  After giving effect to the special allocations
set forth in Section 1 of Exhibit C of the Partnership Agreement, Net Losses
otherwise allocable to the General Partners pursuant to the last sentence of
Sections 6.1.B of the Partnership Agreement shall be allocated:

                 (i)      first, to the General Partners in an amount equal to
                 the excess of (a) the amount of Recourse Liabilities over (b)
                 the Aggregate Protected Amount;

                 (ii)     next, to and among the Protected Partners, in
                 proportion to their respective Protected Amounts, until such
                 time as the Protected Partners have been allocated cumulative
                 Net Losses pursuant to this clause (ii) equal to the Aggregate
                 Protected Amount; and

                 (iii)    thereafter, to the General Partners.

         8.      AMENDMENTS.  Notwithstanding any provision in the Partnership
Agreement to the contrary, the provisions of this Amendment may be waived or
amended or otherwise modified with the prior written consent of holders (being
either the Equity Holders and/or the Indirect Equity Holders) of more than
seventy-five percent (75%) of the OP Units at the time outstanding and without
the consent of any other Limited Partner.

         9.      TIME IS OF THE ESSENCE.  Time is of the essence of each and
                 every provision of this Amendment.

         10.     DEFINED TERMS.  All capitalized terms used in this Amendment
and not otherwise defined shall have the meanings assigned to them in the
Partnership Agreement.  Except as modified herein, all terms and conditions of
the Partnership Agreement shall remain in full force and effect, which terms
and conditions the General Partner hereby ratifies and affirms.




                                       6
<PAGE>   7


         IN WITNESS WHEREOF, the undersigned has executed this Amendment as of
the date first set forth above.

                                      EQUITY OFFICE PROPERTIES TRUST, a Maryland
                                      real estate investment trust, a General
                                      Partner of EOP Operating Limited
                                      Partnership and on behalf of existing
                                      Limited Partners



                                      By:        /s/ STANLEY M. STEVENS
                                                 -------------------------------
                                      Name:      Stanley M. Stevens
                                      Title:     Executive Vice President


                              CONTRIBUTOR:

                              PALO ALTO SQUARE LIMITED PARTNERSHIP, an Illinois
                              limited partnership, by its general partner:

                                       Samuel Zell, Robert Lurie General
                                       Partners, an Illinois partnership, by one
                                       of its general partners:

                                               Zell General Partners, Inc., an
                                               Illinois corporation

                                               By:    /s/ DONALD J. LIEBENTRITT
                                                      --------------------------
                                               Name:  Donald J. Liebentritt
                                               Title: Executive Vice President




                                       7
<PAGE>   8

SIGNATURES CONTINUED



                                                   EQUITY HOLDERS:



                                                   /s/ ADRIENNE NASSAU
                                                   -----------------------------
                                                   Adrienne Nassau
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ AN
                                                                 ---------------




                                                   ANDA Partnership


                                                   By:   /s/ MARK SLEVAK
                                                         -----------------------
                                                         Its: Trustee
                                                             -------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ MS
                                                                 ---------------





                                       8

<PAGE>   9

SIGNATURES CONTINUED





                                                   /s/ EDWARD H. SCHWARTZ
                                                   -----------------------------
                                                   Edward H. Schwartz
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:
                                                                 ---------------




                                                   Estate of Sylvia K. Grossman


                                                   By:   /s/
                                                         -----------------------
                                                         Its:
                                                              ------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:
                                                                 ---------------





                                       9
<PAGE>   10

SIGNATURES CONTINUED



                                      Henry A. Proesel Descendants Trusts
                                      Partnership


                                      By:   /s/ MARGARET E. MCNAMARA, Co-Trustee
                                            ------------------------------------
                                            Its:  Partner
                                                  ------------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE: /s/ MEM
                                                    ----------------------------




                                      Hugh M. Hefner 1991 Trust


                                      By:      /s/ HUGH HEFNER
                                               ---------------------------------
                                               Its:
                                                    ----------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE:
                                                     ---------------------------




                                      Isabelle Kole Stein Trust


                                      By:      /s/ ISABELLE KOLE STEIN
                                               ---------------------------------
                                               Its:
                                                    ----------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE: /s/ IKS
                                                    ----------------------------





                                      10
<PAGE>   11

SIGNATURES CONTINUED



                                      James V. Proesel Marital Exempt Trust


                                      By:   /s/ MARGARET E. MCNAMARA, Co-Trustee
                                            ------------------------------------
                                            Its:  Co-Trustee
                                                --------------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE:  /s/ MEM
                                                     ---------------------------




                                       /s/ JUDITH MOLINSKY
                                      ------------------------------------------
                                      Judith Molinsky
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE:  /s/ JM
                                                     ---------------------------





                                       11
<PAGE>   12

SIGNATURES CONTINUED



                                                   LFT Partnership


                                                   By:  /s/ ANN LURIE
                                                        ------------------------
                                                        Its:  Trustee
                                                              ------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ AL
                                                                 ---------------




                                                   /s/ MORTON H. FRIEDMAN
                                                   -----------------------------
                                                   Morton H. Friedman
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ MHF
                                                                  --------------




                                                   /s/ MORTON S. GROSSMAN
                                                   -----------------------------
                                                   Morton S. Grossman
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ MSG
                                                                  --------------





                                       12
<PAGE>   13

SIGNATURES CONTINUED



                                                   Robert H. & Ann Lurie Trust



                                                   By: /s/ MARK SLEZAK
                                                       -------------------------
                                                       Its: Trustee
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ MS
                                                                 ---------------




                                                   Robert Scot Bldg Venture


                                                   By: /s/ ROBERT HARRIS
                                                       -------------------------
                                                       Its: TTEE
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ RH
                                                                 ---------------


                                                  RSB Properties Trust


                                                   By: /s/
                                                       -------------------------
                                                       Its: Trustee
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/
                                                                 ---------------

                                                  Seiler Trust UTA dated 3/15/95

                                                   By: /s/ DONALD H. SEILER
                                                       -------------------------
                                                       Its: Trustee
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ DS
                                                                 ---------------



                                       13
<PAGE>   14

SIGNATURES CONTINUED



                                    Seiler Family Revocable Trust dated 9/18/90


                                    By:  /s/ STUART W. SEILER
                                         ---------------------------------------
                                         Its: Trustee
                                         ---------------------------------------
                                    TO BECOME A PROTECTED PARTNER
                                    INITIAL HERE: /s/ SWS
                                                  ------------------------------




                                    Helen S. Fredkin Revocable Trust

                                    By:  /s/ HELEN S. FREDKIN REV. TRUST
                                         ---------------------------------------
                                         Its:  Trustee
                                              ----------------------------------
                                    TO BECOME A PROTECTED PARTNER
                                    INITIAL HERE: /s/ HSF
                                                  ------------------------------





                                       14
<PAGE>   15

SIGNATURES CONTINUED



                                                   Sgm & Pin Trust



                                                By:   /s/ STUART MOLDAW
                                                      --------------------------
                                                      Its:
                                                           ---------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ SM
                                                                  --------------




                                                   Steven Kole Trust


                                                By:   /s/ STEVEN M. KOLE
                                                      --------------------------
                                                      Its: Trustee
                                                           ---------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ SMK
                                                                  --------------


                                                By:   /s/ SUSAN HANDELMAN MILLER
                                                      --------------------------
                                                      Susan Handelman Miller
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ SHM
                                                                 ---------------

                                                By:   /s/ VICTOR A. LOWNES
                                                      --------------------------
                                                      Victor A. Lownes, III
                                                   TO BECOME A PROTECTED PARTNER
                                                    INITIAL HERE: /s/ VAL
                                                                  --------------






                                       15
<PAGE>   16

SIGNATURES CONTINUED


                                     Samstock/Alpha, L.L.C.



                                     By: /s/ DONALD J. LIEBENTRITT
                                         ---------------------------------------
                                     Its: Vice President
                                         ---------------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------




                                     Samstock/SZRT, L.L.C.


                                     By:   /s/ DONALD J. LIEBENTRITT
                                           -------------------------------------
                                           Its:  Vice President
                                                --------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------



                                     Samstock/ZFT, L.L.C.



                                     By: /s/ DONALD J. LIEBENTRITT
                                         ---------------------------------------
                                     Its: Vice President
                                         ---------------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------



                                     Samstock/ZGPI, L.L.C.



                                     By: /s/ DONALD J. LIEBENTRITT
                                         ---------------------------------------
                                     Its: Vice President
                                         ---------------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------



                                       16