How to call or override the XML in magento? I created a module and want to call its XML before add to cart.How i do this please explain?
For "add to cart" it is using checkout.xml and for another xml \app\design\frontend\base\default\layout\prescriptionform.xml
<?xml version="1.0"?>
<layout version="0.1.0">
<prescriptionform_index_index>
<reference name="root">
<action method="setTemplate"><template>page/1column.phtml</template></action>
</reference>
<reference name="content">
<block type="prescriptionform/index" name="prescriptionform_index" template="prescriptionform/index.phtml"/>
</reference>
</prescriptionform_index_index>
</layout>
app\code\local\Louisoptical\Prescriptionform\etc\config.xml
<?xml version="1.0"?>
<config>
<modules>
<Louisoptical_Prescriptionform>
<version>0.1.0</version>
</Louisoptical_Prescriptionform>
</modules>
<frontend>
<routers>
<prescriptionform>
<use>standard</use>
<args>
<module>Louisoptical_Prescriptionform</module>
<frontName>prescriptionform</frontName>
</args>
</prescriptionform>
</routers>
<layout>
<updates>
<prescriptionform>
<file>prescriptionform.xml</file>
</prescriptionform>
</updates>
</layout>
</frontend>
<global>
<helpers>
<prescriptionform>
<class>Louisoptical_Prescriptionform_Helper</class>
</prescriptionform>
</helpers>
<blocks>
<prescriptionform>
<class>Louisoptical_Prescriptionform_Block</class>
</prescriptionform>
</blocks>
<models>
<prescriptionform>
<class>Louisoptical_Prescriptionform_Model</class>
<resourceModel>prescriptionform_mysql4</resourceModel>
</prescriptionform>
<prescriptionform_mysql4>
<class>Louisoptical_Prescriptionform_Model_Mysql4</class>
<entities>
<prescription>
<table>prescription</table>
</prescription>
</entities>
</prescriptionform_mysql4>
</models>
<resources>
<prescriptionform_setup>
<setup>
<module>Louisoptical_Prescriptionform</module>
</setup>
<connection>
<use>core_setup</use>
</connection>
</prescriptionform_setup>
<prescriptionform_write>
<connection>
<use>core_write</use>
</connection>
</prescriptionform_write>
<prescriptionform_read>
<connection>
<use>core_read</use>
</connection>
</prescriptionform_read>
</resources>
</global>
<admin>
<routers>
<prescriptionform>
<use>admin</use>
<args>
<module>Louisoptical_Prescriptionform</module>
<frontName>prescriptionform</frontName>
</args>
</prescriptionform>
</routers>
</admin>
<adminhtml>
<menu>
<prescriptionform module="prescriptionform">
<title>Prescriptionform</title>
<sort_order>100</sort_order>
<children>
<prescriptionformbackend module="prescriptionform">
<title>Prescription Form</title>
<sort_order>0</sort_order>
<action>prescriptionform/adminhtml_prescriptionformbackend</action>
</prescriptionformbackend>
<prescription module="prescriptionform">
<title>Manage Prescription</title>
<sort_order>0</sort_order>
<action>prescriptionform/adminhtml_prescription</action>
</prescription>
</children>
</prescriptionform>
</menu>
<acl>
<resources>
<all>
<title>Allow Everything</title>
</all>
<admin>
<children>
<prescriptionform translate="title" module="prescriptionform">
<title>Prescriptionform</title>
<sort_order>1000</sort_order>
<children>
<prescriptionformbackend translate="title">
<title>Prescription Form</title>
</prescriptionformbackend>
<prescription translate="title">
<title>Manage Prescription</title>
</prescription>
</children>
</prescriptionform>
</children>
</admin>
</resources>
</acl>
<layout>
<updates>
<prescriptionform>
<file>prescriptionform.xml</file>
</prescriptionform>
</updates>
</layout>
</adminhtml>
</config>
app\design\frontend\base\default\layout\checkout.xml
<layout version="0.1.0">
<!--
Default layout, loads most of the pages
-->
<default>
<!-- Mage_Checkout -->
<reference name="top.links">
<block type="checkout/links" name="checkout_cart_link">
<action method="addCartLink"></action>
<action method="addCheckoutLink"></action>
</block>
</reference>
<reference name="right">
<block type="checkout/cart_sidebar" name="cart_sidebar" template="checkout/cart/sidebar.phtml" before="-">
<action method="addItemRender"><type>simple</type><block>checkout/cart_item_renderer</block><template>checkout/cart/sidebar/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/cart/sidebar/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/cart/sidebar/default.phtml</template></action>
<block type="core/text_list" name="cart_sidebar.extra_actions" as="extra_actions" translate="label" module="checkout">
<label>Shopping Cart Sidebar Extra Actions</label>
</block>
</block>
</reference>
</default>
<checkout_cart_index translate="label">
<label>Shopping Cart</label>
<remove name="right"/>
<remove name="left"/>
<!-- Mage_Checkout -->
<reference name="root">
<action method="setTemplate"><template>page/1column.phtml</template></action>
</reference>
<reference name="content">
<block type="code/local/Louisoptical/Prescriptionform" before="cart_sidebar" name="prescriptionform" template="prescriptionform/index.phtml"/>
<block type="checkout/cart" name="checkout.cart">
<action method="setCartTemplate"><value>checkout/cart.phtml</value></action>
<action method="setEmptyTemplate"><value>checkout/cart/noItems.phtml</value></action>
<action method="chooseTemplate"/>
<action method="addItemRender"><type>simple</type><block>checkout/cart_item_renderer</block><template>checkout/cart/item/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/cart/item/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/cart/item/default.phtml</template></action>
<block type="core/text_list" name="checkout.cart.top_methods" as="top_methods" translate="label">
<label>Payment Methods Before Checkout Button</label>
<block type="checkout/onepage_link" name="checkout.cart.methods.onepage" template="checkout/onepage/link.phtml"/>
</block>
<block type="page/html_wrapper" name="checkout.cart.form.before" as="form_before" translate="label">
<label>Shopping Cart Form Before</label>
</block>
<block type="core/text_list" name="checkout.cart.methods" as="methods" translate="label">
<label>Payment Methods After Checkout Button</label>
<block type="checkout/onepage_link" name="checkout.cart.methods.onepage" template="checkout/onepage/link.phtml"/>
<block type="checkout/multishipping_link" name="checkout.cart.methods.multishipping" template="checkout/multishipping/link.phtml"/>
</block>
<block type="checkout/cart_coupon" name="checkout.cart.coupon" as="coupon" template="checkout/cart/coupon.phtml"/>
<block type="checkout/cart_shipping" name="checkout.cart.shipping" as="shipping" template="checkout/cart/shipping.phtml"/>
<block type="checkout/cart_crosssell" name="checkout.cart.crosssell" as="crosssell" template="checkout/cart/crosssell.phtml"/>
<block type="checkout/cart_totals" name="checkout.cart.totals" as="totals" template="checkout/cart/totals.phtml"/>
</block>
</reference>
<block type="core/text_list" name="additional.product.info" translate="label">
<label>Additional Product Info</label>
</block>
</checkout_cart_index>
<checkout_cart_configure translate="label">
<label>Configure Cart Item</label>
<update handle="catalog_product_view"/>
<reference name="product.info">
<block type="checkout/cart_item_configure" name="checkout.cart.item.configure.block"></block>
</reference>
</checkout_cart_configure>
<!--
Multi address shipping checkout main layout,
will be rendered on all checkout pages
-->
<checkout_multishipping translate="label">
<label>Multishipping Checkout</label>
<!-- Mage_Checkout -->
<remove name="right"/>
<remove name="left"/>
<reference name="root">
<action method="setTemplate"><template>page/1column.phtml</template></action>
</reference>
<reference name="content">
<block type="checkout/multishipping_state" name="checkout_state" template="checkout/multishipping/state.phtml"/>
</reference>
</checkout_multishipping>
<checkout_multishipping_login>
<update handle="customer_account_login"/>
</checkout_multishipping_login>
<checkout_multishipping_register>
<update handle="customer_account_create"/>
</checkout_multishipping_register>
<!--
Multi address shipping checkout selection of address per item page
-->
<checkout_multishipping_address_select translate="label">
<label>Multishipping Checkout Shipping Address Selection</label>
<update handle="checkout_multishipping"/>
<!-- Mage_Checkout -->
<reference name="content">
<block type="checkout/multishipping_address_select" name="checkout_address_select" template="checkout/multishipping/address/select.phtml"/>
</reference>
</checkout_multishipping_address_select>
<checkout_multishipping_address_selectbilling translate="label">
<label>Multishipping Checkout Billing Address Selection</label>
<update handle="checkout_multishipping"/>
<!-- Mage_Checkout -->
<reference name="content">
<block type="checkout/multishipping_address_select" name="checkout_address_select" template="checkout/multishipping/address/select.phtml"/>
</reference>
</checkout_multishipping_address_selectbilling>
<checkout_multishipping_address_newshipping translate="label">
<label>Multishipping Checkout Shipping Address Creation</label>
<update handle="checkout_multishipping"/>
<update handle="checkout_multishipping_customer_address"/>
</checkout_multishipping_address_newshipping>
<checkout_multishipping_address_newbilling translate="label">
<label>Multishipping Checkout Billing Address Creation</label>
<update handle="checkout_multishipping"/>
<update handle="checkout_multishipping_customer_address"/>
</checkout_multishipping_address_newbilling>
<checkout_multishipping_address_editshipping translate="label">
<label>Multishipping Checkout Shipping Address Edit Form</label>
<update handle="checkout_multishipping"/>
<update handle="checkout_multishipping_customer_address"/>
</checkout_multishipping_address_editshipping>
<checkout_multishipping_address_editaddress>
<update handle="checkout_multishipping"/>
<update handle="checkout_multishipping_customer_address"/>
</checkout_multishipping_address_editaddress>
<checkout_multishipping_address_editbilling translate="label">
<label>Multishipping Checkout Billing Address Edit Form</label>
<update handle="checkout_multishipping"/>
<update handle="checkout_multishipping_customer_address"/>
</checkout_multishipping_address_editbilling>
<checkout_multishipping_customer_address translate="label">
<label>Multishipping Checkout Customer Address Edit Form</label>
<reference name="content">
<block type="customer/address_edit" name="customer_address_edit" template="customer/address/edit.phtml"/>
</reference>
</checkout_multishipping_customer_address>
<!--
Multi address shipping checkout address page
-->
<checkout_multishipping_addresses translate="label">
<label>Multishipping Checkout Address (Any) Form</label>
<update handle="checkout_multishipping"/>
<!-- Mage_Checkout -->
<reference name="content">
<block type="checkout/multishipping_addresses" name="checkout_addresses" template="checkout/multishipping/addresses.phtml">
<action method="addItemRender"><type>default</type><block>checkout/cart_item_renderer</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/multishipping/item/default.phtml</template></action>
</block>
</reference>
</checkout_multishipping_addresses>
<!--
Multi address shipping checkout shipping information
-->
<checkout_multishipping_shipping translate="label">
<label>Multishipping Checkout Shipping Information Step</label>
<update handle="checkout_multishipping"/>
<!-- Mage_Checkout -->
<reference name="content">
<block type="checkout/multishipping_shipping" name="checkout_shipping" template="checkout/multishipping/shipping.phtml">
<action method="addItemRender"><type>default</type><block>checkout/cart_item_renderer</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/multishipping/item/default.phtml</template></action>
<block type="checkout/multishipping_billing_items" name="checkout_billing_items" template="checkout/multishipping/billing/items.phtml">
<action method="addItemRender"><type>default</type><block>checkout/cart_item_renderer</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/multishipping/item/default.phtml</template></action>
</block>
</block>
</reference>
</checkout_multishipping_shipping>
<!--
Multi address shipping checkout billing information
-->
<checkout_multishipping_billing translate="label">
<label>Multishipping Checkout Billing Information Step</label>
<update handle="checkout_multishipping"/>
<!-- Mage_Checkout -->
<reference name="content">
<block type="checkout/multishipping_billing" name="checkout_billing" template="checkout/multishipping/billing.phtml">
<action method="setMethodFormTemplate"><method>purchaseorder</method><template>payment/form/purchaseorder.phtml</template></action>
<!--<block type="checkout/multishipping_billing_items" name="checkout_billing_items" template="checkout/multishipping/billing/items.phtml">
<action method="addItemRender"><type>default</type><block>checkout/cart_item_renderer</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/multishipping/item/default.phtml</template></action>
</block>-->
</block>
</reference>
</checkout_multishipping_billing>
<!--
Multi address shipping checkout overview
-->
<checkout_multishipping_overview translate="label">
<label>Multishipping Checkout Overview</label>
<update handle="checkout_multishipping"/>
<!-- Mage_Checkout -->
<reference name="content">
<block type="checkout/multishipping_overview" name="checkout_overview" template="checkout/multishipping/overview.phtml">
<action method="addItemRender"><type>default</type><block>checkout/cart_item_renderer</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/multishipping/item/default.phtml</template></action>
<action method="addRowItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/multishipping/overview/item.phtml</template></action>
<action method="addRowItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/multishipping/overview/item.phtml</template></action>
<block type="checkout/multishipping_payment_info" name="payment_info">
<action method="setInfoTemplate"><method></method><template></template></action>
</block>
<block type="checkout/agreements" name="checkout.multishipping.agreements" as="agreements" template="checkout/multishipping/agreements.phtml"/>
<block type="checkout/cart_totals" name="totals" />
<block type="core/text_list" name="checkout.multishipping.overview.items.after" as="items_after" translate="label">
<label>Overview Items After</label>
</block>
</block>
</reference>
</checkout_multishipping_overview>
<!--
Multi address shipping checkout success
-->
<checkout_multishipping_success translate="label">
<label>Multishipping Checkout Success</label>
<update handle="checkout_multishipping"/>
<!-- Mage_Checkout -->
<reference name="content">
<block type="checkout/multishipping_success" name="checkout_success" template="checkout/multishipping/success.phtml"/>
</reference>
</checkout_multishipping_success>
<!--
One page checkout main layout
-->
<checkout_onepage_index translate="label">
<label>One Page Checkout</label>
<!-- Mage_Checkout -->
<remove name="left"/>
<reference name="root">
<action method="setTemplate"><template>page/2columns-right.phtml</template></action>
</reference>
<reference name="right">
<action method="unsetChildren"></action>
<block type="page/html_wrapper" name="checkout.progress.wrapper" translate="label">
<label>Checkout Progress Wrapper</label>
<action method="setElementId"><value>checkout-progress-wrapper</value></action>
<block type="checkout/onepage_progress" name="checkout.progress" before="-" template="checkout/onepage/progress.phtml"/>
</block>
</reference>
<reference name="content">
<block type="checkout/onepage" name="checkout.onepage" template="checkout/onepage.phtml">
<block type="checkout/onepage_login" name="checkout.onepage.login" as="login" template="checkout/onepage/login.phtml">
<block type="page/html_wrapper" name="checkout.onepage.login.before" as="login_before" translate="label">
<label>Login/Registration Before</label>
<action method="setMayBeInvisible"><value>1</value></action>
</block>
</block>
<block type="checkout/onepage_billing" name="checkout.onepage.billing" as="billing" template="checkout/onepage/billing.phtml"/>
<block type="checkout/onepage_shipping" name="checkout.onepage.shipping" as="shipping" template="checkout/onepage/shipping.phtml"/>
<block type="checkout/onepage_shipping_method" name="checkout.onepage.shipping_method" as="shipping_method" template="checkout/onepage/shipping_method.phtml">
<block type="checkout/onepage_shipping_method_available" name="checkout.onepage.shipping_method.available" as="available" template="checkout/onepage/shipping_method/available.phtml"/>
<block type="checkout/onepage_shipping_method_additional" name="checkout.onepage.shipping_method.additional" as="additional" template="checkout/onepage/shipping_method/additional.phtml"/>
</block>
<block type="checkout/onepage_payment" name="checkout.onepage.payment" as="payment" template="checkout/onepage/payment.phtml">
<block type="checkout/onepage_payment_methods" name="checkout.payment.methods" as="methods" template="checkout/onepage/payment/methods.phtml">
<action method="setMethodFormTemplate"><method>purchaseorder</method><template>payment/form/purchaseorder.phtml</template></action>
</block>
</block>
<block type="checkout/onepage_review" name="checkout.onepage.review" as="review" template="checkout/onepage/review.phtml"/>
</block>
</reference>
</checkout_onepage_index>
<!--
One page checkout progress block
-->
<checkout_onepage_progress>
<!-- Mage_Checkout -->
<remove name="right"/>
<remove name="left"/>
<block type="checkout/onepage_progress" name="root" output="toHtml" template="checkout/onepage/progress.phtml">
<block type="checkout/onepage_payment_info" name="payment_info">
<action method="setInfoTemplate"><method></method><template></template></action>
</block>
</block>
</checkout_onepage_progress>
<!--
One page checkout payment methods block
-->
<checkout_onepage_paymentmethod>
<remove name="right"/>
<remove name="left"/>
<block type="checkout/onepage_payment_methods" name="root" output="toHtml" template="checkout/onepage/payment/methods.phtml">
<action method="setMethodFormTemplate"><method>purchaseorder</method><template>payment/form/purchaseorder.phtml</template></action>
</block>
</checkout_onepage_paymentmethod>
<!--
One page checkout shipping methods block
-->
<checkout_onepage_shippingmethod>
<!-- Mage_Checkout -->
<remove name="right"/>
<remove name="left"/>
<block type="checkout/onepage_shipping_method_available" name="root" output="toHtml" template="checkout/onepage/shipping_method/available.phtml"/>
</checkout_onepage_shippingmethod>
<checkout_onepage_additional>
<!-- Mage_Checkout -->
<remove name="right"/>
<remove name="left"/>
<block type="checkout/onepage_shipping_method_additional" name="root" output="toHtml" template="checkout/onepage/shipping_method/additional.phtml">
<action method="setDontDisplayContainer"><param>1</param></action>
</block>
</checkout_onepage_additional>
<!--
One page checkout order review block
-->
<checkout_onepage_review translate="label">
<label>One Page Checkout Overview</label>
<!-- Mage_Checkout -->
<remove name="right"/>
<remove name="left"/>
<block type="checkout/onepage_review_info" name="root" output="toHtml" template="checkout/onepage/review/info.phtml">
<action method="addItemRender"><type>default</type><block>checkout/cart_item_renderer</block><template>checkout/onepage/review/item.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/onepage/review/item.phtml</template></action>
<action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/onepage/review/item.phtml</template></action>
<block type="checkout/cart_totals" name="checkout.onepage.review.info.totals" as="totals" template="checkout/onepage/review/totals.phtml"/>
<block type="core/text_list" name="checkout.onepage.review.info.items.before" as="items_before" translate="label">
<label>Items Before</label>
</block>
<block type="core/text_list" name="checkout.onepage.review.info.items.after" as="items_after" translate="label">
<label>Items After</label>
</block>
<block type="checkout/agreements" name="checkout.onepage.agreements" as="agreements" template="checkout/onepage/agreements.phtml"/>
<block type="core/template" name="checkout.onepage.review.button" as="button" template="checkout/onepage/review/button.phtml"/>
</block>
</checkout_onepage_review>
<checkout_onepage_success translate="label">
<label>One Page Checkout Success</label>
<reference name="root">
<action method="setTemplate"><template>page/2columns-right.phtml</template></action>
</reference>
<reference name="content">
<block type="checkout/onepage_success" name="checkout.success" template="checkout/success.phtml"/>
</reference>
</checkout_onepage_success>
<checkout_onepage_failure translate="label">
<label>One Page Checkout Failure</label>
<reference name="root">
<action method="setTemplate"><template>page/2columns-right.phtml</template></action>
</reference>
<reference name="content">
<block type="checkout/onepage_failure" name="checkout.failure" template="checkout/onepage/failure.phtml"/>
</reference>
</checkout_onepage_failure>
<block type="core/list" name="additional.product.info"/>
</layout>
