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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 _combine_out_files(chr_files, work_dir, data):
 """Concatenate all CNV calls into a single file. """

    out_file = "%s.bed" % sshared.outname_from_inputs(chr_files)
    if not utils.file_exists(out_file):
        with file_transaction(data, out_file) as tx_out_file:
            with open(tx_out_file, "w") as out_handle:
                for chr_file in chr_files:
                    with open(chr_file) as in_handle:
                        is_empty = in_handle.readline().startswith("track name=empty")
                    if not is_empty:
                        with open(chr_file) as in_handle:
                            shutil.copyfileobj(in_handle, out_handle)
    return out_file