override - Extend Onepage Billing block in Magento -

i hope knows how solve i've been trying couple of days, followed countless tutorials , sadly not got far.

i attempting override block file : /app/code/core/mage/checkout/block/onepage/billing.php in custom module (for magento 1.7)

in module config.xml file, have added following:

<global>     <blocks>         <checkout>             <rewrite>                 <onepage_billing>                     jrc_giftlistshipping_block_onepage_billing                 </onepage_billing>             </rewrite>         </checkout>     </blocks> </global> 

and created file in following location:

/app/code/local/jrc/giftlistshipping/block/onepage/billing.php contains following simple code:

<?php class jrc_giftlistshipping_block_onepage_billing extends mage_checkout_block_onepage_billing {     protected function _construct()     {         parent::_construct();     } } 

however above generates exception:

exception 'mage_core_exception' message 'invalid block type: jrc_giftlistshipping_block_onepage_billing

i can't see wrong code above hope else can , educate me little

<onepage_billing>jrc_giftlistshipping_block_onepage_billing</onepage_billing> 

don't use reformat code xml in magento