Document:

<PAGE>   1

                               [INTELLICORP LOGO]

                            1975 EL CAMINO REAL WEST
                          MOUNTAIN VIEW, CA 94040-2216

                                                                    Exhibit 10.6

                                                                  March 30, 1999

Wechsler & Co., Inc.
105 South Bedford Road, suite 310
Mount Kisco, N.Y. 10549

Attn: Mr. Norman J. Wechsler

Re:     PURCHASE AND SALE OF COMMON STOCK

Dear Mr. Wechsler:

        This letter documents our agreement regarding the purchase and sale of
shares of Common Stock of IntelliCorp, Inc. (the "Company"). When signed by you
it will constitute a binding agreement between us for the period of one year
from March 30, 1999 until March 30, 2000 (the "Demand Period"). The Company
shall have the right to sell to you (and to require you to purchase) shares of
Company Common Stock, on the terms set forth below, which right shall terminate
on expiration of the Demand Period.

        This will confirm that the Company is entitled to sell, and you will
purchase, during the Demand Period, up to a total of $1,000,000 ("Investment
Maximum") in value of Common Stock of the Company in one or more separate
transactions ("Investment"). This agreement, in conjunction with like agreements
separately executed effective March 30, 1999 with Delaware State Employees'
Retirement Fund, Declaration of Trust for Defined Benefit Plans of Zeneca
Holdings Inc., and Declaration of Trust for Defined Benefit Plans of ICI
American Holdings Inc., represent an aggregate maximum financing of $3,000,000
(collectively called the "Financing").

        For your convenience, when the Company decides to exercise its right
under this agreement, the Company will first attempt to arrive at a mutually
agreeable ratio of Investment between you and the other parties in this
financing. If no agreement can be

<PAGE>   2

Purchase and Sale of Common Stock
March 30, 1999
Page 2

reached between you, the Company has the right to demand an Investment mix based
on the proportional share of each party's Investment Maximum compared to the
aggregate combined maximum of $3,000,000 in this Financing.

        When the Company determines that it wishes to sell shares, it will
provide you written notice ("notice date") of its election to exercise its right
under this agreement; the notice shall specify the dollar amount of the shares
to be sold by the Company and purchased by you and the purchase date, which
shall be not more than ten (10) business days after such notice. The purchase
price for any shares purchased by you hereunder shall be equal to $1.50 per
share or 10% above market price (where market price is as defined below),
whichever is greater, up to a maximum price of $3.00 per share. However, if
during the term of this agreement, the Company sells and issues additional
shares of common stock (excluding issuances on exercise of employee or director
stock options or issuance of shares under this agreement or like agreements with
the parties to this Financing) at a price per share less than the purchase price
per share under this agreement, the purchase price for such shares sold to you
shall be adjusted in the same manner as the conversion price is adjusted in the
Company's Convertible Note dated April 19, 1996 (Section 3), and following any
such adjustment, additional common stock shares will be issued to you by the
Company to reflect this adjustment of the purchase price.

        Market price is defined as the average closing price over the five (5)
trading days ending the day prior to the notice date. On the purchase date (or
as soon thereafter as practically possible), you will deliver cash in
immediately available funds in the amount of the purchase price of such shares.
Within five (5) days after the receipt of funds, the Company will deliver to you
a certificate representing the number of shares being purchased by you.

        This letter will also confirm your investment intent with respect to the
purchase and sale of the shares thereunder. You agree and acknowledge that you
are purchasing the shares for your own account, not with a view to resell them,
and that the certificates for any shares will bear restrictive legends to that
effect. Such shares shall also be subject to registration rights in your favor
which will require the Company to file with the SEC an S-3 Registration
Statement (or equivalent) covering your resale of such shares within 3

<PAGE>   3
Purchase and Sale of Common Stock
March 30, 1999
Page 3

months after the end of the demand period, or within 3 months of the last
transaction purchase date, if earlier.

        If the foregoing accurately describes our agreement regarding the
purchase and sale of the Shares, please so indicate by signing and returning
this letter to me.

                                    Sincerely,

                                    INTELLICORP, INC.

                                    By:
                                       --------------------------------
                                    Kenneth A. Czaja
                                    Chief Financial Officer

THE FOREGOING IS HEREBY
AGREED TO AND ACCEPTED:

WECHSLER & CO., INC..

By:
   --------------------------
    Norman J. Wechsler

<PAGE>   4
                                                                    EXHIBIT 10.6
                                                                         Part II

                            [INTELLICORP LETTERHEAD]

                                                                     May 1, 2000

Wechsler & Co., Inc.
105 South Bedford Road, suite 310
Mount Kisco, N.Y. 10549

Attn: Mr. Norman J. Wechsler

Re:     PURCHASE AND SALE OF COMMON STOCK

Dear Mr. Wechsler:

        This letter documents our agreement to amend and extend the Agreement
for the Purchase and Sale of Common Stock dated March 30, 1999, regarding the
purchase and sale of shares of Common Stock of IntelliCorp, Inc. (the
"Company"). When signed by you it will constitute a binding agreement between us
for the period starting March 31, 2000, and ending the earlier of December 31,
2000 or the receipt by the Company of at least $5,000,000 of equity funding
above and beyond what is obtained under this agreement (the "Demand Period").
The Company shall have the right to sell to you (and to require you to purchase)
shares of Company Common Stock, on the terms set forth below, which right shall
terminate on expiration of the Demand Period.

        This will confirm that the Company is entitled to sell, and you will
purchase, during the Demand Period, up to a total of $2,500,000 ("Investment
Maximum") in value of Common Stock of the Company in one or more separate
transactions ("Investment").

        When the Company determines that it wishes to sell shares, it will
provide you written notice ("notice date") of its election to exercise its right
under this agreement; the notice shall specify the dollar amount of the shares
to be sold by the Company and purchased by you and the purchase date, which
shall be not more than ten (10) business days after such notice. The purchase
price for any shares purchased by you hereunder shall be equal to 110% of market
price (where market price is defined below), with a minimum price of $1.00 per
share and a maximum price of $2.00 per share. However, if during the term of
this agreement, the Company sells and issues additional shares of common stock
(excluding issuances on exercise of employee or director stock options or
issuance of shares under this agreement or like agreements with the parties to
this Financing) at a price per share less than the purchase price per share
under this

<PAGE>   5

Purchase and Sale of Common Stock
May 1, 2000
Page 2

agreement, the purchase price for such shares sold to you shall be adjusted in
the same manner as the conversion price is adjusted in the Company's Convertible
Note dated April 19, 1996 (Section 3), and following any such adjustment,
additional common stock shares will be issued to you by the Company to reflect
this adjustment of the purchase price.

        Market price is defined as the average closing price over the five (5)
trading days ending the day prior to the notice date. On the purchase date (or
as soon thereafter as practically possible), you will deliver cash in
immediately available funds in the amount of the purchase price of such shares.
Within five (5) days after the receipt of funds, the Company will deliver to you
a certificate representing the number of shares being purchased by you.

        As common stock is purchased by you under this agreement, you will also
receive from the Company a five year warrant to purchase additional common stock
of the Company where the number of shares covered by the warrant is equal to 25%
of the number of shares purchased by you in a transaction under this agreement.
The exercise price of the warrant will be 110% of the purchase price for the
shares purchased.

        Finally, this letter will confirm your investment intent with respect to
the purchase and sale of the shares thereunder. You agree and acknowledge that
you are purchasing the shares for your own account, not with a view to resell
them, and that the certificates for any shares will bear restrictive legends to
that effect. Such shares as well as the shares underlying the warrants shall
also be subject to demand registration rights in your favor which, if such
demand is exercised by you, will cause the Company to use its best efforts to
file with the SEC as soon as possible an S-3 Registration Statement (or
equivalent) covering your resale of such shares.

        If the foregoing accurately describes our agreement regarding the
purchase and sale of the Shares, please so indicate by signing and returning
this letter to me.

                                   Sincerely,

                                   INTELLICORP, INC.

                                   By: /s/ KENNETH A. CZAJA
                                      ---------------------------------
                                      Kenneth A. Czaja
                                      Chief Financial Officer

THE FOREGOING IS HEREBY
AGREED TO AND ACCEPTED:

WECHSLER & CO., INC.

<PAGE>   6

Purchase and Sale of Common Stock
May 1, 2000
Page 3

By: /s/ NORMAN J. WECHSLER
   -------------------------------
    Norman J. Wechsler

<PAGE>   7

                                                                    EXHIBIT 10.6
                                                                        Part III

                            [INTELLICORP LETTERHEAD]

                                                                   June 28, 2000

Wechsler & Co., Inc.
105 South Bedford Road, suite 310
Mount Kisco, N.Y. 10549

Attn: Mr. Norman J. Wechsler

Re:     PURCHASE AND SALE OF COMMON STOCK

Dear Mr. Wechsler:

        This letter documents our agreement to amend the Agreement for the
Purchase and Sale of Common Stock dated May 1, 1999, ("Agreement"), regarding
the purchase and sale of shares of Common Stock of IntelliCorp. Inc. (the
"Company"). When signed by you it will constitute a binding agreement between us
to increase the maximum amount that the Company is entitled to sell, and you to
purchase, during the Demand Period (as defined in the Agreement) by $300,000,
from $2,500,000 to $2,800,000. All the other terms and conditions of the
Agreement remain unchanged and apply to this additional amount.

        If the foregoing accurately describes our agreement to amend the May 1
Agreement, please so indicate by signing and returning this letter to me.

                                   Sincerely,

                                   INTELLICORP, INC.

                                   By: /S/ JEROME KLAJBOR
                                      -----------------------------------------
                                      Jerome Klajbor, Chief Financial Officer

THE FOREGOING IS HEREBY
AGREED TO AND ACCEPTED:

WECHSLER & CO., INC.

By: /s/ NORMAN J. WECHSLER
   -------------------------------
    Norman J. Wechsler<PAGE>   1

                                                                    Exhibit 10.7

THESE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
SECURITIES ACT OF 1933, AS AMENDED. SUCH SECURITIES MAY NOT BE TRANSFERRED,
UNLESS A REGISTRATION STATEMENT UNDER THE ACT IS IN EFFECT AS TO SUCH TRANSFER
OR, IN THE OPINION OF COUNSEL TO THE ISSUER, AN EXEMPTION FROM REGISTRATION IS
THEN AVAILABLE.

                                     WARRANT

             VOID AFTER 5:00 P.M., California Time, on June 19, 2005

                        WARRANT TO PURCHASE COMMON STOCK
                                       OF
                                INTELLICORP, INC.

        This is to certify that subject to the terms and conditions hereof, FOR
VALUE RECEIVED, Wechsler & Co., Inc. (the "Initial Holder") or registered
assigns (collectively referred to as the "Holder") is entitled to purchase, at
an exercise price per share of $1.84525 (the "Exercise Price"), 149,032 shares
(the "Warrant Shares") of the Common Stock (the "Common Stock") of IntelliCorp,
Inc., a Delaware corporation (the "Company"), at any time during the period from
June 19, 2000 (the "Commencement Date") to 5:00 P.M., California Time, on June
19, 2005, at which time this Warrant will expire and become void. The following
terms shall apply to this Warrant:

        1. Exercise of Warrant, Reservation of Shares.

               1.1. Subject to the terms and conditions hereof, this Warrant may
be exercised in whole or in part at any time and from time to time on or after
the Commencement Date, and before 5:00 P.M., California Time, on June 19, 2005,
or if such day is a day on which federal or state chartered bank institutions
located in the State of California are authorized by law to close, then on the
next succeeding day which shall not be such a day, by presentation and surrender
hereof to the Company at its principal office or at the office of its warrant
transfer agent, if any, with the attached Purchase Form duly executed and
accompanied by payment, in cash or certified or official bank check payable to
the order of the Company, of the Exercise Price for the number of Warrant Shares
specified in such form. If this Warrant should be exercised in part only, the
Company will, upon presentation of this Warrant upon such exercise, execute and
deliver a new warrant, dated the date hereof, evidencing the rights of the
Holder thereof to purchase the balance of the Warrant Shares purchasable
hereunder upon the same terms and conditions as herein set forth. Upon and as of
such receipt of this Warrant and the Purchase Form by the Company at its office,
in proper form for exercise and accompanied by payment as herein provided, the
Holder shall be deemed to be the holder

<PAGE>   2

of record of the Warrant Shares issuable upon such exercise, notwithstanding
that the stock transfer books of the Company shall then be closed or that
certificates representing the Warrant Shares shall not then be actually
delivered to the Holder. The Company shall promptly take such reasonable steps
as it deems necessary in order to issue the Warrant Shares to be delivered
following exercise of this Warrant.

               1.2. The Company shall at all times after the Commencement Date
and until expiration of this Warrant reserve for issuance and delivery upon
exercise of this Warrant the number of Warrant Shares as shall be required for
issuance and delivery upon exercise of this Warrant.

        2. Fractional Shares. No fractional shares or scrip representing
fractional shares shall be issued upon the exercise of this Warrant. With
respect to any fractional shares called for upon exercise hereof, the Company
will pay to the Holder an amount in cash equal to such fraction multiplied by
the fair market value of a share of Common Stock, as determined by the Board of
Directors.

        3. Transfer in Compliance with the Securities Act of 1933; Exchange,
Assignment or Loss of Warrant.

               3.1. This Warrant may not be assigned or transferred, except as
provided herein, and in accordance with and subject to the provisions of the
Securities Act of 1933, as amended, and the Rules and Regulations promulgated
thereunder (said Act and such Rules and Regulations being hereinafter
collectively referred to as the "Act"). Any purported transfer or assignment
made other than in accordance with this Section 3 shall be null and void and of
no force and effect.

               3.2. This Warrant or the Warrant Shares may not be sold or
otherwise disposed of except as follows:

                      (a) To a person who, in the opinion of counsel reasonably
satisfactory to the Company, is a person to whom this Warrant or the Warrant
Shares may be legally transferred without registration and without the delivery
of a current prospectus under the Act, as well as applicable state securities
laws with respect thereto and then only against receipt of an agreement of such
person to comply with the provisions of this Section 3.2 with respect to any
resale or other disposition of such securities unless, in the opinion of counsel
to the Company, such agreement is not required; or

                      (b) Upon delivery of a prospectus or offering circular
then meeting the requirements of the Act as well as applicable state securities
laws relating to such securities and the offering thereof for such sale or
disposition.

               3.3. Each certificate for Warrant Shares or for any other
security issued or issuable upon exercise of this Warrant shall contain a legend
on the face, in form and

                                      -2-
<PAGE>   3

substance satisfactory to counsel for the Company, setting forth the
restrictions on transfer contained in Section 3.1, unless, in the opinion of
counsel reasonably satisfactory to the Company, such legend is not required.

               3.4. Each holder of the Warrant, the Warrant Shares and any other
security issued or issuable upon exercise of this Warrant shall indemnify and
hold harmless the Company, its directors and officers, and each other person, if
any, who controls the Company against any losses, claims, damages or
liabilities, joint or several, to which the Company or any such director,
officer or any such person may become subject under the Act, any applicable
state securities law or any other statute or at common law, insofar as such
losses, claims, damages or liabilities (or actions in respect thereof) directly
arise out of or are based upon the disposition by such holder of the Warrant,
Warrant Shares or other such securities in violation of the above
representation.

               3.5. Subject to the provisions of Sections 3.1 through 3.4, this
Warrant is exchangeable, without expense, at the option of the Holder, for other
warrants of different denominations entitling the Holder to purchase in the
aggregate the same number of Warrant Shares purchasable on the same terms and
conditions, upon presentation at the principal office of the Company or at the
office of its warrant transfer agent, if any, together with a written notice
signed by the Holder specifying the names and denominations in which new
warrants are to be issued, and may be divided or combined with other warrants
which carry the same rights, upon presentation at the principal office of the
Company or at the office of its warrant transfer agent, if any, together with a
written notice signed by the Holder specifying the names and denominations in
which new warrants are to be issued.

               3.6. Any assignment permitted under this Warrant will be made by
surrender of this Warrant to the Company at its principal office or at the
office of its warrant transfer agent, if any, with the attached Assignment Form
duly executed and accompanied by funds sufficient to pay any transfer tax. In
such event the Company will, without charge, execute and deliver a new Warrant
in the name of the assignee named in such instrument of assignment, and this
Warrant will promptly be canceled.

               3.7. Upon receipt by the Company of evidence satisfactory to it
of the loss, theft, destruction or mutilation of this Warrant, and (in the case
of loss, theft or destruction) of reasonably satisfactory indemnification, and
upon surrender and cancellation of this Warrant, if mutilated, the Company will
execute and deliver a new Warrant of like tenor and date and any such lost,
stolen, or destroyed Warrant shall thereupon become void.

        4. Adjustment of Number of Warrant Shares and Exercise Price.

                                      -3-
<PAGE>   4

               4.1. The number of Warrant Shares for which this Warrant may be
exercised shall be subject to adjustment as follows:

                      (a) In the event there is a subdivision or combination of
the outstanding shares of Common Stock into a larger or smaller number of
shares, the number of Warrant Shares shall be increased or reduced in the same
proportion as the increase or decrease in the outstanding shares of Common
Stock.

                      (b) If the Company declares a dividend on Common Stock
payable in Common Stock or securities convertible into Common Stock, the number
of Warrant Shares shall be increased, as of the record date for determining
which holders of Common Stock shall be entitled to receive such dividend, in
proportion to the increase in the number of outstanding shares of Common Stock
as a result of such dividend.

               4.2. In the event at any time prior to the expiration of this
Warrant of any reorganization or reclassification of the outstanding shares of
Common Stock (other than a change in par value, or from no par value to par
value, or from par value to no par value, or as a result of a subdivision or
combination) or any consolidation or merger of the Company with another entity,
or sale, lease or transfer of all or substantially all of the property or assets
of the Company, the Holder shall have the right, but not the obligation, to
exercise this Warrant. Upon such exercise, the Holder shall have the right to
receive the same kind and number of shares of capital stock and other
securities, cash or other property as would have been distributed to the Holder
upon such reorganization, reclassification, consolidation or merger had the
Holder exercised this Warrant immediately prior to such reorganization,
reclassification, consolidation or merger. The Holder shall pay upon such
exercise the Exercise Price that otherwise would have been payable pursuant to
the terms of this Warrant. If any such reorganization, reclassification,
consolidation or merger results in a cash distribution in excess of the Exercise
Price provided by this Warrant, the Holder may, at the Holder's option, exercise
this Warrant without making payment of the Exercise Price, and in such case the
Company shall, upon distribution to the Holder, consider the Exercise Price to
have been paid in full, and in making settlement to the Holder, shall deduct an
amount equal to the Exercise Price from the amount payable to the Holder.

               4.3. If the Company shall, at any time prior to the expiration of
this Warrant, dissolve, liquidate or wind up its affairs, the Holder shall have
the right, but not the obligation, to exercise this Warrant. Upon such exercise
the Holder shall have the right to receive, in lieu of the shares of Common
Stock that the Holder otherwise would have been entitled to receive, the same
kind and amount of assets as would have been issued, distributed or paid to the
Holder upon any such dissolution, liquidation or winding up with respect to such
shares of Common Stock had the Holder been the holder of record of such shares
of Common Stock receivable upon exercise of this Warrant on the date for
determining those entitled to receive any such distribution. If any such
dissolution,

                                      -4-
<PAGE>   5

liquidation or winding up results in any cash distribution in excess of the
Exercise Price provided for by this Warrant, the Holder may, at the Holder's
option, exercise this Warrant without making payment of the Exercise Price and,
in such case, the Company shall, upon distribution to the Holder, consider the
Exercise Price to have been paid in full, and in making settlement to the Holder
shall deduct an amount equal to the Exercise Price from the amount payable to
the Holder.

               4.4. The Company may retain a firm of independent public
accountants of recognized standing (who may be any such firm regularly employed
by the Company) to make any computation required under this Section 4, and a
certificate signed by such firm shall be conclusive evidence of the correctness
of any computation made under this Section.

               4.5. Whenever the number of shares of Common Stock purchasable
upon the exercise of this Warrant is adjusted as herein provided, the Exercise
Price shall be adjusted by multiplying the applicable Exercise Price immediately
prior to such adjustment by a fraction, the numerator of which shall be the
number of shares of Common Stock purchasable upon exercise of this Warrant
immediately prior to such adjustment and the denominator of which shall be the
number of shares of Common Stock purchasable immediately after such adjustment.

               4.6. Upon any adjustment of the Exercise Price, then and in each
such case the Company shall give written notice thereof, by first class mail,
postage prepaid, addressed to the Holder at the address of such Holder as shown
on the books of the Company, which notice shall state the Exercise Price
resulting from such adjustment, setting forth in reasonable detail the method of
calculation and the facts upon which such calculation is based.

               4.7. The issuance of certificates for shares of Common Stock upon
exercise of this Warrant shall be made without charge to the Holder thereof for
any issuance tax in respect thereof; provided, that the Company shall not be
required to pay any tax which may be payable in respect of any transfer involved
in the issuance and delivery of any certificate in a name other than that of the
Holder.

        5. Rights of Holder. Except as otherwise provided in Section 1.1 above,
this Warrant does not entitle the Holder to any rights of a shareholder of the
Company either at law or in equity, and the rights of any such Holder are
limited to those expressed in this Warrant and are not enforceable against the
Company, except to the extent set forth herein.

        6. Warrant Transfer Agent. Any reference in this Warrant to the warrant
transfer agent will apply if, and only if, the Company will have advised the
Holder that

                                      -5-
<PAGE>   6

such an agent has been designated as an agency for the transfer or exercise of
this Warrant.

        7. Governing Law. This Warrant shall be construed in accordance with the
laws of the State of California.

        8. Notices. Any notice required hereunder shall be by writing and shall
be given by personal delivery, or United States mail, certified or registered
with return receipt requested, postage prepaid and shall be deemed to be
effective five (5) business days after mailing or on the date of delivery if
delivered personally, at the following addresses, or such other addresses as one
party may from time to time give the other in writing:

               To the Company:      INTELLICORP, INC.
                                    1975 El Camino Real
                                    Mountain View, CA  94040
                                    Attention: President

               To Holder:           At the address set forth below.

        IN WITNESS WHEREOF, the Company has executed this Warrant as of the 19th
day of June, 2000.

                                            INTELLICORP, INC.

                                            By:
                                               ------------------------------

        Initial Holder:

        Name:         Wechsler & Co., Inc.

Address:              105 South Bedford Road, Suite 310
                      Mount Kisco, N.Y.  10549
ATTENTION:            Norman J. Wechsler

                                      -6-
<PAGE>   7

                                 PURCHASE FORM

                                                        Dated:
                                                              ------------------

        The undersigned hereby irrevocably elects to exercise the within Warrant
to the extent of purchasing __________ shares of Common Stock and hereby makes
payment of $__________ in payment of the actual Exercise Price thereof.

                     INSTRUCTIONS FOR REGISTRATION OF STOCK

Name:
               ---------------------------------------------------
                  (Please typewrite or print in block letters)

Address:
               ---------------------------------------------------

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

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

                                            Signature:
                                                      ----------------------

<PAGE>   8

                                 ASSIGNMENT FORM

        FOR VALUE RECEIVED, ____________________ hereby sells, assigns and
transfers unto

Name:
      -----------------------------------------------------------------
               (Please typewrite or print in block letters)

Address:___________________________________________________________________
the right to purchase Common Stock represented by this Warrant to the extent of
_________ shares of Stock and does hereby irrevocably constitute and appoint
____________________, attorney, to transfer the same on the books of the Company
with full power of substitution in the premises.

                                        Signature:
                                                  -----------------------

Dated:
       -----------------------------

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