Document:

Prepared by R.R. Donnelley Financial -- Specimen Stock Certificate

 Exhibit 4.1 
  
 
	 COMMON
 	 	 COMMON
 
	 STOCK
 	 	 STOCK
 

 
 IMPAC 
 IMPAC
Medical Systems, Inc. 
  
 
	 THIS CERTIFICATE IS TRANSFERABLE IN
CANTON, MA, JERSEY CITY, NJ AND NEW YORK, NY
 	 	 INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE
 	  	 CUSIP 45255A 10 4
 

 
  
 
	 THIS CERTIFIES THAT
 	  	  
	  	  	 SEE REVERSE SIDE FOR CERTAIN DEFINITIONS AND A STATEMENT OF RIGHTS, PREFERENCES, AND PRIVILEGES
 
	 IS THE RECORD HOLDER OF
 	  	  

 
  
 FULLY PAID AND NONASSESSABLE SHARES OF THE COMMON STOCK, $.001 PAR VALUE,
OF 
  
 IMPAC Medical Systems, Inc. 
  
 transferable on the books of the Corporation by the record holder hereof in person or by duly authorized attorney upon surrender of this certificate properly endorsed. This
certificate is not valid until countersigned and registered by the Transfer Agent and Registrar. 
 WITNESS the facsimile seal of the
Corporation and the facsimile signatures of its duly authorized officers. 
  
 Dated: 

 
 
	 SECRETARY AND TREASURER
 	 	 [SEAL]
 	 	 CHIEF EXECUTIVE OFFICER AND PRESIDENT
 

 
  
 COUNTERSIGNED AND REGISTERED: 
 EquiServe Trust Company, N.A. 
  
 TRANSFER AGENT AND REGISTRAR 
  

BY     
  
 AUTHORIZED OFFICER 

  
 A statement of the powers, designations, preferences and relative, participating,
optional or other special rights of each class of stock or series thereof and the qualifications, limitations or restrictions of such preferences and/or rights as established, from time to time, by the Certificate of Incorporation of the Corporation
and by any certificate of determination, the number of shares constituting each class and series, and the designations thereof, may be obtained by the holder hereof upon request and without charge at the principal office of the Corporation.

  
 The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as
though they were written out in full according to applicable laws or regulations: 
  
 
	 TEN COM  —  as tenants in common
 	  	 UNIF GIFT MIN ACT—..........................Custodian......................
 
	 TEN ENT   —  as tenants by the entireties
 	  	                                       
          (Cust)                           
 (Minor)
 
	 JT TEN      —  as joint tenants with right of
 	  	                                       
    under Uniform Gifts to Minors
 
	                          survivorship and not as tenants in
 	  	                                       
    Act..........................................................
 
	                                   common
 	  	                 (State)
 
	  	  	 UNIF TRF MIN ACT — ....................Custodian (until age........)
 
	  	  	                                       
          (Cust)
                                       
    ........................under Uniform Transfers
                                       
        (Minor)
 
	 
	  	  	                                       
 to Minors Act...................................
 
	  	  	                                       
          (State)
 

 
  
 Additional abbreviations may also be used though not
in the above list. 
  
 FOR VALUE RECEIVED,
                                        
                                        
            hereby sell, assign and transfer unto 
  
 PLEASE INSERT SOCIAL SECURITY OR OTHER 
 IDENTIFYING NUMBER OF ASSIGNEE 
  
 [                                ] 
  
  
 
	                                      
                                        
                                       
                                        
                                        
                   
 
	 (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

	  
	                                      
                                        
                                        
                                        
                                        
                 
 
	  
	  
	                                      
                                        
                                        
                                        
                                        
    Shares
 
	 of the common stock represented by the within Certificate, and do hereby irrevocably constitute and appoint

	  
	                                      
                                        
                                       
                                        
                                        
  Attorney
 
	  
	 to transfer the said stock on the books of the within named Corporation with full power of substitution in the
premises.
 
	  
	  
	 Dated______________________ 
 
	  
	  
	  

 
 
	  	 	  
	  	 	 X___________________________
 
	 
	  	 	 X___________________________
 
	 NOTICE:
 	 	 THE SIGNATURE(S) TO THIS ASSIGNMENT MUSTCORRESPOND WITH THE NAME(S) AS WRITTEN UPON THE  FACE OF THE CERTIFICATE IN EVERY PARTICULAR, WITHOUT ALTERATION
OR ENLARGEMENT OR ANY CHANGE WHATEVER.
 

 
 Signature(s) Guaranteed 
  
 By_________________________________________ 
  
 THE
SIGNATURE(S) MUST BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION (BANKS, STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT TO S.E.C. RULE 17Ad-15.<PAGE>

                                                                   EXHIBIT 10.21

--------------------------------------------------------------------------------

                           WAIVER AND THIRD AMENDMENT
                               TO CREDIT AGREEMENT

                                     BETWEEN

                           CONTANGO OIL & GAS COMPANY

                                       AND

                               GUARANTY BANK, FSB
                                    AS LENDER

                         Effective as of April 26, 2002

                          -----------------------------

             REDUCING REVOLVING LINE OF CREDIT OF UP TO $50,000,000

                          -----------------------------

--------------------------------------------------------------------------------

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                PAGE
                                                                ----
<S>                                                            <C>
ARTICLE I   DEFINITIONS ...................................... 1
 1.01       Terms Defined Above .............................. 1
 1.02       Terms Defined in Agreement ....................... 1
 1.03       References ....................................... 1
 1.04       Articles and Sections ............................ 1
 1.05       Number and Gender ................................ 1
ARTICLE II  WAIVERS .......................................... 2
 2.01       Waivers .......................................... 2
 2.02       Limitation on Waivers ............................ 2
ARTICLE III AMENDMENTS ....................................... 2
 3.01       Amendment of Section 1.2 ......................... 2
 3.02       Amendment of Section 2.6(a) ...................... 2
 3.03       Amendment of Section 6.8 ......................... 2
 3.04       Amendment of Section 6.12 ........................ 2
 3.05       Deletion of Section 6.15 ......................... 3
 3.06       Amendment of Exhibit III ......................... 3
ARTICLE IV  CONDITIONS ....................................... 3
 4.01       Receipt of Documents ............................. 3
 4.02       Accuracy of Representations and Warranties ....... 3
 4.03       Matters Satisfactory to Lender ................... 3
ARTICLE V   REPRESENTATIONS AND WARRANTIES ................... 3
ARTICLE VI  RATIFICATION ..................................... 3
ARTICLE VII MISCELLANEOUS .................................... 3
 7.01       Scope of Amendment ............................... 3
 7.02       Agreement as Amended ............................. 4
 7.03       Parties in Interest .............................. 4
 7.04       Rights of Third Parties .......................... 4
 7.05       ENTIRE AGREEMENT ................................. 4
 7.06       GOVERNING LAW .................................... 4
 7.07       JURISDICTION AND VENUE ........................... 4
</TABLE>

                                        i

<PAGE>

                 WAIVER AND THIRD AMENDMENT TO CREDIT AGREEMENT

     This WAIVER AND THIRD AMENDMENT TO CREDIT AGREEMENT (this "Third
Amendment") is made and entered into effective as of April 26, 2002, between
CONTANGO OIL & GAS COMPANY, a Delaware corporation, (the "Borrower"), and
GUARANTY BANK, FSB, a federal savings bank (the "Lender").

                               W I T N E S S E T H

     WHEREAS, the above named parties did execute and exchange counterparts of
that certain Credit Agreement dated June 29, 2001, as further amended by First
Amendment to Credit Agreement dated January 8, 2002, and as further amended by
Second Amendment to Credit Agreement dated February 13, 2002 (the "Agreement"),
to which reference is here made for all purposes;

     WHEREAS, the parties subject to and bound by the Agreement are desirous of
amending the Agreement in the particulars hereinafter set forth;

     NOW, THEREFORE, in consideration of the mutual covenants and agreements of
the parties to the Agreement, as set forth therein, and the mutual covenants and
agreements of the parties hereto, as set forth in this Third Amendment, the
parties hereto agree as follows:

                                    ARTICLE I
                                   DEFINITIONS

     1.01  Terms Defined Above. As used herein, each of the terms "Agreement,"
"Borrower," "Lender," and "Third Amendment" shall have the meaning assigned to
such term hereinabove.

     1.02  Terms Defined in Agreement. As used herein, each term defined in the
Agreement shall have the meaning assigned thereto in the Agreement, unless
expressly provided herein to the contrary.

     1.03  References. References in this Third Amendment to Article or Section
numbers shall be to Articles and Sections of this Third Amendment, unless
expressly stated herein to the contrary. References in this Third Amendment to
"hereby," "herein," hereinafter," hereinabove," "hereinbelow," "hereof," and
"hereunder" shall be to this Third Amendment in its entirety and not only to the
particular Article or Section in which such reference appears.

     1.04  Articles and Sections. This Third Amendment, for convenience only,
has been divided into Articles and Sections and it is understood that the
rights, powers, privileges, duties, and other legal relations of the parties
hereto shall be determined from this Third Amendment as an entirety and without
regard to such division into Articles and Sections and without regard to
headings prefixed to such Articles and Sections.

     1.05  Number and Gender. Whenever the context requires, reference herein
made to the single number shall be understood to include the plural and likewise
the plural shall be understood to include the singular. Words denoting sex shall
be construed to include the masculine, feminine, and

                                        1

<PAGE>

neuter, when such construction is appropriate, and specific enumeration shall
not exclude the general, but shall be construed as cumulative. Definitions of
terms defined in the singular and plural shall be equally applicable to the
plural or singular, as the case may be.

                                   ARTICLE II
                                     WAIVERS

     2.01  Waivers. The Lenders hereby waive any Default or Event of Default
arising under the Agreement or any other Loan Document solely as a result of
violations of Sections 6.12, 6.13, and 6.15 for the quarter ending March 31,
2002.

     2.02  Limitation on Waivers. The scope of the waivers set forth in Section
2.1 are expressly limited to their terms and do not extend to any other or
future breaches, Defaults, violations or Events of Default under the Agreement
or any other Loan Document.

                                   ARTICLE III
                                   AMENDMENTS

     The Borrower and the Lender hereby amend the Agreement in the following
particulars:

     3.01  Amendment of Section 1.2. Section 1.2 of the Agreement is amended to
read as follows:

     The following definition shall be amending as follows:

     "Debt Service" shall mean an amount equal to the actual interest payments
     for the quarter, plus 8.3% of the outstanding balance of this facility at
     the end of the quarter.

     3.02  Amendment of Section 2.6(a). Section 2.6(a) of the Agreement is
amended to read as follows:

     "2.6  Borrowing Base Determinations. (a) The Borrowing Base as of April 1,
     2002, is acknowledged by the Borrower and the Lender to be $22,000,000.
     Commencing on May 1, 2002, and continuing thereafter on the first day of
     each calendar month through the next Borrowing Base review, the amount of
     the Borrowing Base shall be reduced by $475,000."

     3.03  Amendment of Section 6.8. Section 6.8 of the Agreement is hereby
amended to add as a last sentence the following:

     "6.8  Dividends and Distributions. ... Notwithstanding the above, the
     Borrower may issue convertible preferred stock in an amount of $15,000,000
     to $20,000,000 in June of 2002 that will carry a coupon rate between 7% to
     9% annually and pay dividends quarterly."

     3.04  Amendment of Section 6.12. Section 6.12 of the Agreement is hereby
amended to read as follows:

                                        2

<PAGE>

     "6.12 Current Ratio. Permit, as of the close of any fiscal quarter, the
     ratio of Current Assets to Current Liabilities to be less than 1.25 to
     1.00."

     3.05  Deletion of Section 6.15. Section 6.15 is hereby deleted from the
Agreement.

     3.06  Amendment of Exhibit III. Exhibit III, i.e. the "Form of Compliance
Certificate" shall be as set forth on Exhibit III to this Third Amendment.

                                   ARTICLE IV
                                   CONDITIONS

     The obligation of the Lender to amend the Agreement as provided herein is
subject to the fulfillment of the following conditions precedent:

     4.01  Receipt of Documents. The Lender shall have received, reviewed, and
approved the following documents and other items, appropriately executed when
necessary and in form and substance satisfactory to the Lender:

     (a)   multiple counterparts of this Third Amendment, as requested by the
           Lender; and

     (b)   such other agreements, documents, items, instruments, opinions,
           certificates, waivers, consents, and evidence as the Lender may
           reasonably request.

     4.02  Accuracy of Representations and Warranties. The representations and
warranties contained in Article IV of the Agreement and this Third Amendment
shall be true and correct.

     4.03  Matters Satisfactory to Lender. All matters incident to the
consummation of the transactions contemplated hereby shall be satisfactory to
the Lender.

                                    ARTICLE V
                         REPRESENTATIONS AND WARRANTIES

     The Borrower hereby expressly re-makes, in favor of the Lender, all of the
representations and warranties set forth in Article IV of the Agreement, and
represents and warrants that all such representations and warranties remain true
and unbreached.

                                   ARTICLE VI
                                  RATIFICATION

     Each of the parties hereto does hereby adopt, ratify, and confirm the
Agreement and the other Loan Documents, in all things in accordance with the
terms and provisions thereof, as amended by this Third Amendment.

                                   ARTICLE VII
                                  MISCELLANEOUS

     7.01  Scope of Amendment. The scope of this Third Amendment is expressly
limited to the matters addressed herein and this Third Amendment shall not
operate as a waiver of any past,

                                        3

<PAGE>

present, or future breach, Default, or Event of Default under the Agreement.
except to the extent, if any, that any such breach, Default, or Event of Default
is remedied by the effect of this Third Amendment.

     7.02  Agreement as Amended. All references to the Agreement in any document
heretofore or hereafter executed in connection with the transactions
contemplated in the Agreement shall be deemed to refer to the Agreement as
amended by this Third Amendment.

     7.03  Parties in Interest. All provisions of this Third Amendment shall be
binding upon and shall inure to the benefit of the Borrower, the Lender and
their respective successors and assigns.

     7.04  Rights of Third Parties. All provisions herein are imposed solely and
exclusively for the benefit of the Lender and the Borrower, and no other Person
shall have standing to require satisfaction of such provisions in accordance
with their terms and any or all of such provisions may be freely waived in whole
or in part by the Lender at any time if in its sole discretion it deems it
advisable to do so.

     7.05  ENTIRE AGREEMENT. THIS THIRD AMENDMENT CONSTITUTES THE ENTIRE
AGREEMENT BETWEEN THE PARTIES HERETO WITH RESPECT TO THE SUBJECT HEREOF AND
SUPERSEDES ANY PRIOR AGREEMENT, WHETHER WRITTEN OR ORAL, BETWEEN SUCH PARTIES
REGARDING THE SUBJECT HEREOF. FURTHERMORE IN THIS REGARD, THIS THIRD AMENDMENT,
THE AGREEMENT, THE NOTE, THE SECURITY INSTRUMENTS, AND THE OTHER WRITTEN
DOCUMENTS REFERRED TO IN THE AGREEMENT OR EXECUTED IN CONNECTION WITH OR AS
SECURITY FOR THE NOTE REPRESENT, COLLECTIVELY, THE FINAL AGREEMENT AMONG THE
PARTIES THERETO AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR,
CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE ARE NO
UNWRITTEN ORAL AGREEMENTS AMONG THE PARTIES.

     7.06  GOVERNING LAW. THIS THIRD AMENDMENT, THE AGREEMENT AND THE NOTE SHALL
BE DEEMED TO BE CONTRACTS MADE UNDER AND SHALL BE CONSTRUED IN ACCORDANCE WITH
AND GOVERNED BY THE LAWS OF THE STATE OF TEXAS. THE PARTIES ACKNOWLEDGE AND
AGREE THAT THIS AGREEMENT AND THE NOTE AND THE TRANSACTIONS CONTEMPLATED HEREBY
BEAR A NORMAL, REASONABLE, AND SUBSTANTIAL RELATIONSHIP TO THE STATE OF TEXAS.

     7.07  JURISDICTION AND VENUE. ALL ACTIONS OR PROCEEDINGS WITH RESPECT TO,
ARISING DIRECTLY OR INDIRECTLY IN CONNECTION WITH, OUT OF, RELATED TO, OR FROM
THIS THIRD AMENDMENT, THE AGREEMENT OR ANY OTHER LOAN DOCUMENT MAY BE LITIGATED
IN COURTS HAVING SITUS IN HARRIS COUNTY, TEXAS. EACH OF THE BORROWER AND THE
LENDER HEREBY SUBMITS TO THE JURISDICTION OF ANY LOCAL, STATE, OR FEDERAL COURT
LOCATED IN HARRIS COUNTY, TEXAS, AND HEREBY WAIVES ANY RIGHTS IT MAY HAVE TO
TRANSFER OR CHANGE THE JURISDICTION OR VENUE OF ANY

                                        4

<PAGE>

LITIGATION BROUGHT AGAINST IT BY THE BORROWER OR THE LENDER IN ACCORDANCE WITH
THIS SECTION.

     IN WITNESS WHEREOF, this Third Amendment to Credit Agreement is executed
effective the date first hereinabove written.

                           BORROWER

                           CONTANGO OIL & GAS COMPANY

                           By: /s/ WILLIAM H. GIBBONS
                               ------------------------------------
                                   William H. Gibbons
                                   Vice President and Treasurer

                                        5

<PAGE>

                                  LENDER

                                  GUARANTY BANK, FSB

                                  By: /s/ MICHAEL ANSOLABEHERE
                                      -----------------------------
                                      Michael Ansolabehere
                                      Vice President

                                        6

<PAGE>

                                   EXHIBIT III

                        [FORM OF COMPLIANCE CERTIFICATE]

                                 ________, 2002

Guaranty Bank, FSB
333 Clay Street, Suite 4430
Houston, Texas 77002
Attention: A.R. Gralla, Jr.

     Re:   Credit Agreement dated as of June 29, 2001, by and between CONTANGO
           OIL & GAS COMPANY and GUARANTY BANK, FSB (as amended, restated, or
           supplemented from time to time, the "Credit Agreement")

Ladies and Gentlemen:

     Pursuant to applicable requirements of the Credit Agreement, the
undersigned, as a Responsible Officer of the Borrower, hereby certifies to you
the following information as true and correct as of the date hereof or for the
period indicated, as the case may be:

     [1.   To the best of the knowledge of the undersigned, no Default or Event
     of Default exists as of the date hereof or has occurred since the date of
     our previous certification to you, if any.

     1.    To the best of the knowledge of the undersigned, the following
     Defaults or Events of Default exist as of the date hereof or have occurred
     since the date of our previous certification to you, if any, and the
     actions set forth below are being taken to remedy such circumstances:]

     2.    The compliance of the Borrower with the financial covenants of the
     Credit Agreement, as of the close of business on _________________ , is
     evidenced by the following:

     (a)   Section 6.12: Current Ratio. Permit, as of the close of any fiscal
     quarter, the ratio of Current Assets to Current Liabilities to be less than
     1.25 to 1.00.

                                     Actual

     (b)   Section 6.13: Debt Coverage Ratio. Permit, as of the close of any
     fiscal quarter, the ratio of (a) quarterly EBITDAX to (b) Debt Service to
     be less than 2.50 to 1.0.

                                     Actual

                                        1

<PAGE>

     (c)   Section 6.14: Funded Debt Ratio. Permit, as of the close of any
     fiscal quarter, the ratio of (a) Total Funded Debt to (b) annualized
     quarterly EBITDAX to be more than 1.00 to 1.00.

                                     Actual

     (d)   Section 6.16. General and Administrative Expenses. Permit, as of the
     close of any fiscal year, general and administrative expenses to exceed
     $3,300,000.

                                     Actual

     3.    No Material Adverse Effect has occurred since the date of the
     Financial Statements dated as of ______________.

     Each capitalized term used but not defined herein shall have the meaning
assigned to such term in the Credit Agreement.

                                          Very truly yours,

                                          CONTANGO OIL & GAS COMPANY

                                          By: __________________________________

                                          Printed Name: ________________________

                                          Title: _______________________________

                                        2

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