code: def combine(in_files, out_file, config):
      
      if not utils.file_exists(out_file):
          with file_transaction(config, out_file) as tx_out_file:
              with open(tx_out_file, "w") as out_handle:
                  for in_file in in_files:
                      with open(in_file) as in_handle:
                          shutil.copyfileobj(in_handle, out_handle)
      return out_file
docs: Combine multiple BED files into a single output.