Need help on xml-schema

Hi,
 I need a help on the following xml to have a schema.
Looked into the followin link but did not make work
http://xsd.stylusstudio.com/2001Aug/post03013.htm

<?xml version="1.0" encoding="UTF-8"?>
<overall_operation>
 <operation type="list_products">
                <ops_description>Listing all the products of a
company</ops_description>
                    <module>powesystem</module>
                          <comp_name>APC</comp_name>
                          <prod_price>50K$</prod_pric>
                          <manf_date>2001</manf_date>
                          <pool_name>Electrical</pool_name>
                          <fail_retry>2</fail_retry>
                        <storage_type>avialble</storage_type>
                        <storage_check>false</storage_check>
                        <api_type>sync</api_type>
                        <product_name>transformer</product_name>
 </operation>
 <operation type="search_product">
                <ops_description>Search the products of a company from the
repository</ops_description>
                    <module>high-voltage</module>
                         <module>powesystem</module>
                          <comp_name>APC</comp_name>
                          <pool_name>Electrical</pool_name>
                          <fail_retry>2</fail_retry>
                        <storage_type>avialble</storage_type>
                        <storage_check>false</storage_check>
                        <api_type>sync</api_type>
                        <product_name>setup-transformer</product_name>
 </operation>
        <oeration>
   .
..
..
..
        </operation>
 </overall_operation>


Here different elements with operation like list_products,search_products
and so on.
Each elements will have some comman attributes such as
ops_description,module and so on.
Also some of the uniquie attributes for each elements such as
prod_price,manf_date etc.
I want to have a xml schema to validate. Some of the attributes also
optional.
I tried using abstract and derived but could not make it work.
Thanks in advance.