One usecase calling another usecase is pretty common however, I have not seen usecases being referred directly in sequence diagrams. What I have seen( and used) is actors being used in seq. diagrams. For instance, you can model a whole subsystem as an actor("payment processing") and use it in the seq. diagram.
If I were you, I will not be using those "rare" UML features and find a way around to represent the same concept in a more standardized way that employs common and easy to understand notations. I am not sure if you have a situation that makes this option inevitable, so I cannot offer you alternatives.
[ May 27, 2003: Message edited by: Ajith Kallambella ]
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Ajith M. Kallambella
SCJP2, SCWCD, IBM-XML Dev,
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Co-author of
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discuss on extends use case