Sequence Diagram

XodoX

I'm having issues doing a sequence diagram for a program.

Like this format here:

http://upload.wikimedia.org/wikipedi...kEmail.svg.png

The scenario is the following:

Placing a bet - a customer places a bet for a game. The customer has to be logged into the system with his ID. He determines the bet ( amount of $ ), the ID of the game, and the expected winner. If the team is not participating, then a error message will be displayed accordingly. Otherwise, it will be checked if the customer has already placed a bet on the game.

Ok, there's more, but I think that's enough for my question.

So you have the customer box and the system box at the top.

So I have -> [ Input_stats] and the second arrow is <- [ Team not participating].

How do I do the 3rd arrow? I don't think you can just put it below the second one. Maybe create an extra box ? It would be <- [ checking_double].
So it's basically one requirement and two possible cases. I'm not sure how to display this in this diagram.