Given a block's verified block txids and a verified transaction, 
        find out where it is in the list of txids (i.e. what's its index)?

def tx_index( cls, verified_block_txids, verified_tx ):
        """
        Given a block's verified block txids and a verified transaction, 
        find out where it is in the list of txids (i.e. what's its index)?
        """
        tx_hash = cls.tx_hash( verified_tx )
        return verified_block_txids.index( tx_hash )