Gates and the Pay for Itinerary sequence diagram

[carlos fernandez]

I have an issue with the "pay for itinerary" SD.

I am using an interaction frame to refer to the "Price Itinerary" SD in the course of the "Prepare Itinerary" SD. This seems straight forward because "Price Itinerary" has a single input message (some kind of itinerary) and a single output message (some kind of price).

On the other hand the "Pay for Itinerary" SD seems to have at least 3 incoming messages (basic flow):
- the itinerary that is going to be paid for
- the customers "CC"
- the CC to use for payment

The only examples of gates or parameters/return values (for example, the Developerworks articles by Donald Bell) only show a single input and a single output.

How do you package up a more complex SD like "Pay for Itinerary" to use as an interaction diagram in the "Prepare Itinerary" SD?

Do I use the standard gate for an input and output message and just add another lifeline to represent the external context (e.g. some sort of controller or series of screens), passing additional messages into and out of the external context lifeline?

Thanks in advance.

Carlos