code: def _combine_files(orig_files, base_out_file, data, fill_paths=True):
      
      orig_files = [x for x in orig_files if x and utils.file_exists(x)]
      if not orig_files:
          return None
      out_file = "%s-combine%s" % (utils.splitext_plus(base_out_file)[0],
                                      utils.splitext_plus(orig_files[0])[-1])
      with open(out_file, "w") as out_handle:
          for orig_file in orig_files:
              with open(orig_file) as in_handle:
                  for line in in_handle:
                      if fill_paths and line.startswith("file"):
                          line = _fill_file_path(line, data)
                      out_handle.write(line)
              out_handle.write("\n\n")
      return out_file
docs: Combine multiple input files, fixing file paths if needed.
  
      We fill in full paths from files in the data dictionary if we're
      not using basepath (old style GEMINI).