import luisy
    import pandas as pd
    
    @luisy.raw
    @luisy.csv_output(delimiter=',')
    class InputFile(luisy.ExternalTask):
        label = luisy.Parameter()
    
        def get_file_name(self): 
            return f"file_{self.label}"
    
    @luisy.interim
    @luisy.requires(InputFile)
    class ProcessedFile(luisy.Task):
        def run(self):
            df = self.input().read()
            # Some more preprocessings
            # ...
            # Write to disk
            self.write(df)
    
    @luisy.final
    class MergedFile(luisy.ConcatenationTask):
        def requires(self):
            for label in ['a', 'b', 'c', 'd']:
                yield ProcessedFile(label=label)