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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def batch_start(job, input_args):
 """ Downloads shared files that are used by all samples for alignment and places them in the jobstore. """

    shared_files = ['ref.fa', 'ref.fa.amb', 'ref.fa.ann', 'ref.fa.bwt', 'ref.fa.pac', 'ref.fa.sa', 'ref.fa.fai']
    shared_ids = {}
    for fname in shared_files:
        url = input_args[fname]
        shared_ids[fname] = job.addChildJobFn(download_from_url, url, fname).rv()
    job.addFollowOnJobFn(spawn_batch_jobs, shared_ids, input_args)