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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 _merge_doc(original, to_merge):
 # type: (str, str) -> str """Merge two usage strings together. Args: original: The source of headers and initial section lines. to_merge: The source for the additional section lines to append. Returns: A new usage string that contains information from both usage strings. """

    if not original:
        return to_merge or ''
    if not to_merge:
        return original or ''
    sections = []
    for name in ('usage', 'arguments', 'options'):
        sections.append(_merge_section(
            _get_section(name, original),
            _get_section(name, to_merge)
        ))
    return format_usage('\n\n'.join(s for s in sections).rstrip())