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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 restructuredtext(value, settings=RESTRUCTUREDTEXT_SETTINGS):
 """ RestructuredText processing with optionnally custom settings. """

    try:
        from docutils.core import publish_parts
    except ImportError:
        warnings.warn("The Python docutils library isn't installed.",
                      RuntimeWarning)
        return value

    parts = publish_parts(source=force_bytes(value),
                          writer_name='html4css1',
                          settings_overrides=settings)
    return force_text(parts['fragment'])