Are there examples of this elsewhere in the code that I can follow?  
Is it best to check if the value is an object and an instance of WPSC_Purchase_Log:

```
public function submit_claimed_stock( $purchase_log_id ) {
	if ( is_object( $purchase_log_id ) && get_class( $purchase_log_id ) == 'WPSC_Purchase_Log' ) {
		$purchase_log_id = $purchase_log_id->get( 'id' );
	}
	...
```

...or is there a more elegant way of doing this?