Document:

<PAGE>
                                                                    Exhibit 10.2

                                                                    May 21, 2002
Mr. Thomas O. Pyle
One Devonshire Place, Apt #3701
Boston, MA  02109

Dear Tom:

This letter is to confirm that the following options to acquire the Common Stock
of Millipore Corporation granted to you under the Millipore Corporation 1999
Stock Option Plan for Non-Employee Directors (the "99 Option Plan") have been
converted into options to acquire the number of shares of Common Stock, $0.01
par value, of Mykrolis Corporation at the exercise price specified in the table
below. These converted options are subject to the terms of this letter
agreement, of the 99 Option Plan and of the original grant letter with the
exception that Mykrolis Corporation is substituted for Millipore Corporation
and, as specified below, the options cover shares of Mykrolis Corporation.

<TABLE>
<CAPTION>

--------- --------------- --------------- ----------------- ---------------- ----------------
                             Original     Unvested Options  Post Conversion   Post-Conversion
Grant #   Grant Date      Exercise Price     on 2/27/02     Exercise Price   Unvested Options
--------- --------------- --------------- ----------------- ---------------- ----------------
<S>       <C>             <C>             <C>               <C>              <C>
006878    April 16, 1998      $35.625             500           $6.5602            2,715
--------- --------------- --------------- ----------------- ---------------- ----------------
007383    April 22, 1999     $29.9375            1,000          $5.5129            5,431
--------- --------------- --------------- ----------------- ---------------- ----------------
007899    April 27, 2000     $69.9375            1,500         $12.8787            8,146
--------- --------------- --------------- ----------------- ---------------- ----------------
008549    April 25, 2001      $56.30             2,000         $10.3674           10,861
--------- --------------- --------------- ----------------- ---------------- ----------------
</TABLE>

The above conversions have been determined by calculating the relationship (the
"ratio") of the closing price of Millipore common stock on the NYSE on the
spin-off date (February 27, 2002) [$57.02] to the opening price of Mykrolis
common stock on the NYSE on the first trading day after the spin-off (February
28, 2002) [$10.50]. This ratio [5.43047611] was multiplied times the number of
Millipore shares covered by each of your unvested options to yield the number of
Mykrolis shares covered by the post conversion option; the ratio was then
divided into the exercise price of each of your Millipore unvested options to
yield the exercise price of the post conversion option. These calculations were
designed to preserve the "intrinsic value" of your pre-spin Millipore options in
the converted options immediately after the spin. For example, your 1998
unvested option grant had an intrinsic value of $10,697.50 as of the close of
trading on the NYSE on February 27, 2002 [(500 X MIL FMV of $57.02) - (500 X
original exercise price of $35.625) = $10,697.50] and that option had the same
intrinsic value as of the opening of trading on the NYSE on February 28, 2002
[(2,715 X MYK FMV of $10.50) - (2,715 X post conversion exercise price of
$6.5602) = $10,697.50].

<PAGE>
Mr. Thomas O. Pyle
May 21, 2002
Page 2

All of the above converted options will expire on August 9, 2006 in accordance
with and subject to the provisions of the 99 Option Plan. Under the 99 Option
Plan all outstanding stock options normally are cancelled upon the resignation
of a director. There is an exception, however, where the director resigns with
the consent of Millipore Corporation, a special exercise period of up to 5 years
is established. Accordingly, upon your resignation from the Millipore Board of
Directors on August 9, 2001 your unvested Millipore options were held in
suspense until the spin-off date when they were converted into Mykrolis options
as described above.

Your converted options will vest in annual increments on the date of the Annual
Meeting of the Board of Directors of Mykrolis Corporation in accordance with the
following schedule:

<TABLE>
<CAPTION>
--------- --------------- ----------------- ------------------- ------------------- ------------------
                           Vest on Annual    Vest on Annual      Vest on Annual      Vest on Annual
Grant #   Grant Date      Meeting Date 2002 Meeting Date 2003   Meeting Date 2004   Meeting Date 2005
--------- --------------- ----------------- ------------------- ------------------- ------------------
<S>       <C>             <C>               <C>                 <C>                 <C>
006878    April 16, 1998       2,715           Fully Vested        Fully Vested       Fully Vested
--------- --------------- ----------------- ------------------- ------------------- ------------------
007383    April 22, 1999       2,715              2,716            Fully Vested       Fully Vested
--------- --------------- ----------------- ------------------- ------------------- ------------------
007899    April 27, 2000       2,715              2,715               2,716           Fully Vested
--------- --------------- ----------------- ------------------- ------------------- ------------------
008549    April 25, 2001       2,715              2,715               2,715               2,716
--------- --------------- ----------------- ------------------- ------------------- ------------------
</TABLE>

Please acknowledge receipt of this letter by signing and returning the enclosed
copy of this letter to the address below. It is necessary for us to receive this
signed acknowledgement before any of the converted options may be exercised.

                                                Very truly yours,

                                                MYKROLIS CORPORATION

                                               /s/ C. William Zadel
                                     ----------------------------------------
                                                 C. William Zadel

                               Chairman of the Board and Chief Executive Officer

Please return the signed copy of this letter to:

Ms. Gloria Williams
Mykrolis Corporation
129 Concord Road
Billerica, MA 01821

Receipt and acceptance of the foregoing is hereby acknowledged:

          /s/ Thomas O. Pyle                          27 May 02
---------------------------------------          ---------------------
               Signature                                 Date<PAGE>
                                                                    EXHIBIT 10.0

                              EMPLOYMENT AGREEMENT

     This Employment Agreement ("Agreement") is made this 15th day of May, 2002
(the "Agreement Date") between the following parties ("Parties"):

     1. Bank of Tanglewood, National Association (the "Bank");

     2. BOK Financial Corporation, an Oklahoma corporation ("BOKF"); and,

     3. Robert G. Greer, an individual residing in Houston, Texas (the
"Executive").

     The Bank and Executive, in consideration of the promises and covenants set
forth herein (the receipt and adequacy of which are hereby acknowledged) and
intending to be legally bound hereby, agree as follows:

     1. Purpose of This Agreement. The purpose of this Agreement is as follows:

     a. The Bank is a national association organized under the National Bank
Act. The Bank is engaged in the banking business in Houston, Texas.

     b. BOKF is a bank holding company and owns all of the capital stock of Bank
of Texas, National Association ("Bank of Texas"). Bank of Texas is engaged in
the banking business in Dallas and Houston, Texas.

     c. The Executive is currently serving as Chairman of the Board of the Bank.
The Executive currently has no written agreement of employment with the Bank,
but the Executive is currently receiving salary compensation and other benefits
from the Bank (collectively, the "Current Benefits").

     d. Pursuant to an Agreement and Plan of Merger dated May 15, 2002 (the "BHC
Merger Agreement") among BOKF, Bank, and TW Interim Bank ("TW"), BOKF

                                        1

<PAGE>

is acquiring ownership of the Bank and it is anticipated that at some date in
the future the Bank will be merged into Bank of Texas (the "Bank Merger"). The
Closing (as defined in the BHC Merger Agreement) is hereafter referred to as
"the consummation of the Merger Agreement" or the "BHC Merger". Upon and subject
to the BHC Merger, the Bank desires to retain the services of Executive and the
Executive desires to continue to render services to the Bank.

     e. The purpose of this Agreement is to set forth the terms and conditions
(i) on which the Bank shall, subject to consummation of the Merger Agreement,
employ the Executive from and after consummation of the Merger Agreement and
(ii) on which the Executive agrees not to compete with the Bank. As hereafter
used, "Bank" shall mean Bank of Tanglewood, National Association preceding the
Bank Merger and Bank of Texas following the Bank Merger.

     2. Employment. The Bank hereby employs the Executive, and the Executive
hereby agrees to work for the Bank, on the following terms and conditions:

     a. Executive shall serve as Chairman of the Board of the Bank until such
time as the Bank Merger is consummated and as Vice Chairman of the Board Bank of
Texas, National Association following consummation of the Bank Merger, subject
to the direction of the Chief Executive Officer of the Bank.

     b. Executive shall devote all time and attention reasonably necessary to
the affairs of the Bank and shall serve the Bank diligently, loyally, and to the
best of his ability.

     c. Executive shall serve in such other or additional positions as an
officer and/or director of the Bank or any of its affiliates as the Chief
Executive Officer of the Bank may request; provided, however, Executive's
residence and place of work

                                        2

<PAGE>

shall remain in the Houston area.

     d. Notwithstanding anything herein to the contrary, Executive shall not be
precluded from engaging in any charitable, civic, political or community
activity or membership in any professional organization.

     3. Compensation. As the sole, full and complete compensation to the
Executive for the performance of all duties of Executive under this Agreement
and for all services rendered by Executive to the Bank or to any affiliate of
the Bank:

     a. The Bank shall pay to Executive the sum of $106,500 per year during each
year of this Agreement, payable in installments in arrears, less usual and
customary payroll deductions for FICA, federal and state withholding, and the
like, at the times and in the manner in effect in accordance with the usual and
customary payroll policies generally in effect from time to time at the Bank
("Annual Salary"). The Annual Salary shall not be decreased at any time during
the Term (as hereafter defined) of this Agreement.

     b. The Bank shall pay and provide to Executive pension, thrift, medical
insurance, disability insurance plan benefits, and other fringe benefits,
generally in effect for senior executive employees of the Bank and its
affiliates (the "Additional Benefits"). Executive shall be credited with his
prior service at the Bank in BOKF's 401k plan and in connection with the
Additional Benefits (other than in connection with BOKF's pension plan).
Executive shall not be credited with prior service in BOKF's pension plan, but
in the event Executive is not vested at the time Executive's employment with the
Bank is terminated (without cause), the Bank shall pay Executive an amount equal
to the pension plan contributions made by the Bank in respect of Executive plus
any earnings thereon.

                                        3

<PAGE>

     c. The Bank may, from time to time in Bank's sole discretion consistent
with the practices generally in effect for senior executive employees of BOKF
and its affiliates, pay or provide, or agree to pay or provide, Executive a
bonus, stock option, or other incentive or performance based compensation. All
such bonus, stock option or other incentive or performance based compensation,
regardless of its nature (hereinafter called "Performance Compensation") shall
not constitute Annual Salary.

     i. The Bank shall pay (in cash, less usual and customary payroll deductions
for FICA, federal and state withholding, and the like) Executive minimum bonuses
(the "Mandatory Performance Compensation"), as follows:

     (1) for calendar year 2002, payable on or before March 15, 2003, an amount
equal to $45,000 less the amounts paid in by Bank in respect of 2002 on or
before the Closing in accordance with the current bonus accruals being made by
Bank; and,

     (2) for each succeeding year of the Term, on or before March 15 of 2004,
2005, 2006, and 2007, respectively, $45,000.

     d. The Bank shall reimburse Executive for reasonable and necessary
entertainment, travel and other expenses in accordance with BOKF's standard
policies in general effect for senior executive employees of the Bank (which
includes dues for lunch clubs, but does not include reimbursement for country
club memberships or dues).

     e. BOKF shall consider Executive for the award of options to acquire shares
of BOKF Common Stock in respect of the BOKF stock option plan at the time and on
the same terms and conditions as offered generally to the senior executive
officers of the Bank; provided, BOKF shall award Executive options to acquire

                                        4

<PAGE>

not less than three thousand (3,000) shares of BOKF Common Stock in year 2002.

     f. The Executive shall be allowed vacation, holidays, and other employee
benefits not described above in accordance with the Bank's standard policy in
general effect for Bank's senior executive employees.

     g. Subject to the terms and conditions of paragraph 8, Executive hereby
agrees to accept the foregoing compensation in lieu of all Current Benefits and
as the sole, full and complete compensation to Executive for the performance of
all duties of Executive under this Agreement and for all services rendered by
Executive to the Bank or any affiliate of the Bank.

     4. Term of Employment. The term of Executive's employment ("Employment")
pursuant to this Agreement (the "Term") shall commence (the "Commencement") as
of the commencement of the first Bank pay-roll period immediately preceding the
effective date of the Closing of the BHC Merger and shall terminate on the fifth
anniversary date of the Commencement.

     5. Termination of Employment. Notwithstanding the provisions of paragraph 4
of this Agreement, the Employment may be terminated on the following terms and
conditions:

     a. Termination by Bank Without Cause. In the event the Bank terminates
Executive without cause, (A) the Bank shall forthwith upon such termination pay
to Executive his then Annual Salary and any Mandatory Performance Compensation
for the remaining portion of the original five year Term (as if Executive's
Employment with Bank had not been terminated), whether or not Executive seeks or
obtains other employment, in accordance with the usual and customary payroll
policies generally in effect from time to time at Bank, (B) the Executive shall
be

                                        5

<PAGE>

entitled to receive any benefits, insured or otherwise, that Executive would
otherwise be able to receive under any benefit plan of the Bank of which
Executive is a beneficiary in accordance with paragraph 3(b), provided that any
and all unvested options which would otherwise have vested during the Term shall
immediately vest upon the effective date of the termination of Executive's
employment, and (C) Executive shall be entitled to receive those amounts due
Executive pursuant to paragraph 7c and shall be bound by the Non-Competition and
Non-Solicitation Agreements.

     b. Termination by Bank for Cause. The Bank may terminate the Employment for
cause on the following terms and conditions:

     i. The Bank shall be deemed to have cause to terminate Executive's
Employment only in one of the following events:

     (1) The Executive shall willfully fail to substantially perform his
obligations under this Agreement (it being understood that any such failure
resulting from Executive's incapacity due to physical or mental illness shall
not be deemed willful);

     (2) The Executive commits any act which is intended by Executive to
materially injure the Bank;

     (3) The Executive is convicted of any criminal act or act involving moral
turpitude;

     (4) The Executive commits any dishonest or fraudulent act which the Bank
reasonably deems material to the Bank, including the reputation of the Bank; or,

     (5) Any refusal by Executive to obey written orders or instructions of

                                        6

<PAGE>

the Chief Executive Officer of the Bank unless such instructions would require
Executive to commit an illegal act, could subject Executive to personal
liability, would require Executive to violate the terms of this Agreement, or
would otherwise be inconsistent with the duties of an officer of a national
banking association.

     ii. The Bank shall be deemed to have cause to terminate Executive's
Employment only when a majority of the members of the Board of Directors of the
Bank finds that, in the good faith opinion of such majority, the Executive
committed one or more of the acts set forth in clauses (1) through (5) of the
preceding subparagraph, such finding to have been made after at least twenty
(20) business days' notice to the Executive and an opportunity for the
Executive, together with his counsel, to be heard before such majority. The
determination of such majority, made as set forth above, shall be binding upon
the Bank and the Executive;

     iii. The effective date of a termination for cause shall be the date of the
action of such majority finding the termination was with cause. In the event the
Bank terminates Executive's Employment for cause, (A) the Bank shall pay
Executive the Executive's then Annual Salary through, but not beyond, the
effective date of the termination, (B) the Bank shall pay the Executive the
Mandatory Performance Compensation accrued through the date of termination of
Employment, (C) the Executive shall receive those Additional Benefits accrued
through but not beyond the effective date of such termination which are
thereafter payable under the terms and

                                        7

<PAGE>

provisions of benefit plans then in effect in accordance with paragraph 3(b)
above, and (D) Executive shall be entitled to receive those amounts due
Executive pursuant to Paragraph 7c and shall be bound by the provisions of the
Non-Competition Agreement and Non-Solicitation Agreement.

     6. Provisions Respecting Illness and Death. In the event Executive is
unable to perform his duties under this Agreement on a full-time basis for a
period of six (6) consecutive months by reason of illness or other physical or
mental disability, and at or before the end of such period, Executive does not
return to work on a full-time basis, the Bank may terminate Executive's
Employment without further or additional compensation being due the Executive
from the Bank except Annual Salary, the Mandatory Performance Compensation
accrued through the date of termination, and Additional Benefits accrued through
the date of such termination under benefit plans then in effect in accordance
with paragraph 3(b) above. In the event of the death of the Executive, the
Employment of the Executive shall automatically terminate as of the date of
death without further or additional compensation being due the Executive, except
the Bank shall pay to the estate of the Executive the Annual Salary accrued
through the date of termination, the Mandatory Performance Compensation accrued
through the date of termination, and the Additional Benefits accrued through the
date of such termination under benefit plans then in effect in accordance with
paragraph 3(b) above.

     7. Agreement Not to Compete and Solicit. The provisions of this paragraph
are hereafter called the "Non-Competition Agreement" and the "Non-Solicitation
Agreement."

     a. Executive agrees (the "Non-Competition Agreement") that, following any
termination of Employment for whatever cause, for a period of two (2) years
after

                                        8

<PAGE>

the termination of Executive's employment by Bank, Executive shall not directly
or indirectly (whether as an officer, director, employee, partner, stockholder,
creditor or agent, or representative of other persons or entities) engage in the
banking business or in any business in which the Bank or any affiliate of the
Bank has, as of the date of such termination engaged (the "banking business" or
"banking services"), in Harris County, Texas or any county in which Bank of
Texas maintains a banking office for which Executive is assigned supervisory
authority, or in any counties contiguous to any of such counties (the "Trade
Area"), provided that the foregoing shall not apply to ownership by Executive of
up to ten percent (10%) of the common stock of a corporation traded on the
facilities of a national securities exchange engaged in the banking business of
which Executive is not a director, officer, employee, agent or representative.

     b. Executive agrees (the "Non-Solicitation Agreement") that, following any
termination of the Employment for whatever cause, for a period of five (5) years
after the termination, Executive shall not directly or indirectly (whether as an
officer, director, employee, partner, stockholder, creditor or agent, or
representative of other persons or entities) contact or solicit, in any manner
indirectly or directly, individuals or entities who were at anytime during the
Term clients of Bank or Bank's affiliates in the Trade Area for the purpose of
providing banking services or contact or solicit employees of Bank or Bank's
affiliates to seek employment with any person or entity except the Bank and its
affiliates, whether, in either case, such contact or solicitation is made within
or without the Trade Area, provided that the foregoing shall not apply to
ownership by Executive of up to ten percent (10%) of the common stock of a
corporation traded

                                        9

<PAGE>

on the facilities of a national securities exchange engaged in the banking
business of which Executive is not a director, officer, employee, agent or
representative.

     c. The Bank shall:

     i. pay Executive, in addition to any other amounts which may be due
Executive, during each year in which both the Non-Competition Agreement and the
Non-Solicitation Agreement are in effect, $50,000, payable in installments in
arrears, less usual and customary payroll deductions for FICA, federal and state
withholding, and the like, at the times and in the manner in effect in
accordance with the usual and customary payroll policies generally in effect
from time to time at the Bank; and,

     ii. pay Executive, in addition to any other amounts which may be due
Executive, during each year in which only the Non-Solicitation Agreement is in
effect, $25,000, payable in installments in arrears, less usual and customary
payroll deductions for FICA, federal and state withholding, and the like, at the
times and in the manner in effect in accordance with the usual and customary
payroll policies generally in effect from time to time at the Bank.

     d. Executive agrees that (i) the Non-Competition Agreement and the
Non-Solicitation Agreement are entered into in connection with the sale to BOKF
of the goodwill of the business of the Bank to BOKF, (ii) Executive is receiving
contemporaneously herewith the sum of $10,000 as separate additional
consideration for the Non-Competition Agreement and Non-Solicitation Agreement
which consideration Executive agrees is full and fair consideration for

                                       10

<PAGE>

the provisions of the Non-Competition Agreement and the Non-Solicitation
Agreement, (iii) the restrictions imposed upon Executive by this Non-Competition
Agreement and Non-Solicitation Agreement are essential and necessary to ensure
BOKF acquires the goodwill of the Bank, and (iv) all the restrictions (including
particularly the time and geographical limitations) set forth in this
Non-Competition Agreement and Non-Solicitation Agreement are fair and
reasonable.

     e. Executive agrees that (i) any remedy at law for any breach of this
Non-Competition Agreement would be inadequate, (ii) in the event of any breach
of this Non-Competition Agreement or Non-Solicitation Agreement, the
Non-Competition Agreement and Non-Solicitation Agreement shall constitute
incontrovertible evidence of irreparable injury to the Bank, and (iii) the Bank
shall be entitled to both immediate and permanent injunctive relief without the
necessity of establishing or posting any bond therefor to preclude any such
breach (in addition to any remedies of law which the Bank may be entitled).

     8. Condition Precedent. The obligations of the Parties under this Agreement
shall be subject to the condition precedent that the Closing of the BHC Merger
shall have occurred.

     9. Obligations of BOKF. BOKF shall be jointly and severally liable for the
obligations of the Bank arising under this Agreement.

     10. Miscellaneous Provisions. The following miscellaneous provisions shall
apply to this Agreement:

     a. All notices or advices required or permitted to be given by or pursuant
to this Agreement, shall be given in writing. All such notices and advices shall
be (i) delivered personally, (ii) delivered by facsimile or delivered by U.S.
Registered or

                                       11

<PAGE>

Certified Mail, Return Receipt Requested mail, or (iii) delivered for overnight
delivery by a nationally recognized overnight courier service. Such notices and
advices shall be deemed to have been given (i) the first business day following
the date of delivery if delivered personally or by facsimile, (ii) on the third
business day following the date of mailing if mailed by U.S. Registered or
Certified Mail, Return Receipt Requested, or (iii) on the date of receipt if
delivered for overnight delivery by a nationally recognized overnight courier
service. All such notices and advices and all other communications related to
this Agreement shall be given as follows:

                  If to the Bank
                  or BOKF:          BOKF Financial Corporation
                                    5956 Sherry Lane, Suite 1100
                                    Dallas, TX 75225
                                    Attention: C. Fred Ball, Jr.
                                    Telephone No: (214) 706-0336
                                    Telecopy No.: (214) 706-0350

                  With a Copy to:   Frederic Dorwart
                                    Old City Hall
                                    124 East Fourth Street
                                    Tulsa, OK 74103-5010
                                    Telephone No.: (918) 583-9945
                                    Telecopy No.: (918) 583-8251

                  If to Executive:  Robert G. Greer
                                    ________________________
                                    ________________________
                                    Telephone No.: ____________
                                    Telecopy No.: _____________

or to such other address as the party may have furnished to the other parties in
accordance herewith, except that notice of change of addresses shall be
effective only upon receipt.

     b. This Agreement is made and executed in Harris County, Texas and all
actions or

                                       12

<PAGE>

proceedings with respect to, arising directly or indirectly in connection with,
out of, related to or from this Agreement, shall be litigated in courts having
situs in Harris County, Texas.

     c. This Agreement shall be subject to, and interpreted by and in accordance
with, the laws of the State of Texas.

     d. This Agreement is the entire Agreement of the parties respecting the
subject matter hereof. There are no other agreements, representations or
warranties, whether oral or written, respecting the subject matter hereof,
except as stated in this Agreement.

     e. This Agreement, and all the provisions of this Agreement, shall be
deemed drafted by all of the parties hereto.

     f. This Agreement shall not be interpreted strictly for or against any
party, but solely in accordance with the fair meaning of the provisions hereof
to effectuate the purposes and interest of this Agreement.

     g. Each party hereto has entered into this Agreement based solely upon the
agreements, representations and warranties expressly set forth herein and upon
his own knowledge and investigation. Neither party has relied upon any
representation or warranty of any other party hereto except any such
representations or warranties as are expressly set forth herein.

     h. Each of the persons signing below on behalf of a party hereto represents
and warrants that he or she has full requisite power and authority to execute
and deliver this Agreement on behalf of the parties for whom he or she is
signing and to bind such party to the terms and conditions of this Agreement.

     i. This Agreement may be executed in counterparts, each of which shall be
deemed

                                       13

<PAGE>

an original. This Agreement shall become effective only when all of the parties
hereto shall have executed the original or counterpart hereof. This Agreement
may be executed and delivered by a facsimile transmission of a counterpart
signature page hereof.

     j. In any action brought by a party hereto to enforce the obligations of
any other party hereto, the prevailing party shall be entitled to collect from
the opposing party to such action such party's reasonable litigation costs and
attorneys fees and expenses (including court costs, reasonable fees of
accountants and experts, and other expenses incidental to the litigation).

     k. This Agreement shall be binding upon and shall inure to the benefit of
the parties and their respective heirs, administrators, successors and assigns.
In particular, Executive's rights and Bank's obligations under paragraphs 3b,
3c, 5a, 5b, and 6 shall survive the death of the Executive and the termination
of this Agreement.

     l. This is not a third party beneficiary contract, except BOKF (including
each affiliate thereof) shall be a third party beneficiary of this Agreement.
Subject to paragraph 10k, no person or entity other than a party signing this
Agreement and those designated as a third party beneficiary herein shall have
any rights under this Agreement.

     m. This Agreement may be amended or modified only in a writing, as agreed
to by the parties hereto, which specifically references this Agreement.

     n. A party to this Agreement may decide or fail to require full or timely
performance of any obligation arising under this Agreement. The decision or
failure of a party hereto to require full or timely performance of any
obligation arising under this Agreement (whether on a single occasion or on
multiple occasions) shall not be

                                       14

<PAGE>

deemed a waiver of any such obligation. No such decisions or failures shall give
rise to any claim of estoppel, laches, course of dealing, amendment of this
Agreement by course of dealing, or other defense of any nature to any obligation
arising hereunder.

     o. In the event any provision of this Agreement, or the application of such
provision to any person or set of circumstances, shall be determined to be
invalid, unlawful, or unenforceable to any extent for any reason, the remainder
of this Agreement, and the application of such provision to persons or
circumstances other than those as to which it is determined to be invalid,
unlawful, or unenforceable, shall not be affected and shall continue to be
enforceable to the fullest extent permitted by law.

Dated and effective the date first set forth above.

                                    BANK OF TANGLEWOOD, NATIONAL ASSOCIATION

                               By     /s/ Richard W. Jochetz
                                    -------------------------------------------

                                    BOK FINANCIAL CORPORATION

                               By     /s/ James F. Ulrich
                                    -------------------------------------------

                                      /s/ Robert G. Greer
                                    -------------------------------------------
                                    Robert G. Greer

                                       15

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