In building my clieop3 module together with uc_payment_direct_debit I ran into the missing of documentation about the callback parameter $op of hook_payment.
That was a bummer. Because the drupal development process is still a lot of reverse engineering. Having read Pro Drupal Develpment eases the pain a little. Finding ones way studying other modules is ok if it's not on a daily basis. I must be doing something wrong.
The good side is that this $op gets documented now.