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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 _format_download_uri_for_extension(etextno, extension, mirror=None):
 """Returns the download location on the Project Gutenberg servers for a given text and extension. The list of available extensions for a given text can be found via the formaturi metadata extractor. """

    mirror = mirror or _GUTENBERG_MIRROR
    root = mirror.strip().rstrip('/')
    path = _etextno_to_uri_subdirectory(etextno)

    uri = '{root}/{path}/{etextno}{extension}'.format(
        root=root,
        path=path,
        etextno=etextno,
        extension=extension)

    return uri