[galaxy-dev] How to build an application that uses a set of files?

Hello, I would like to get advice how to implement the following application into Galaxy: We have a set of say, 17 tabular files, we would like to present them to the User and allow him to select a subset of them, say 9. Once he selected these files we would like to invoke our program “merge.py” against the subset of 9 files the User selected whose result is an output of one file. How would the xml look? Thanks! George Weingart