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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 DocbookSlidesPdf(env, target, source=None, *args, **kw):
 """ A pseudo-Builder, providing a Docbook toolchain for PDF slides output. """

    # Init list of targets/sources
    target, source = __extend_targets_sources(target, source)

    # Init XSL stylesheet
    __init_xsl_stylesheet(kw, env, '$DOCBOOK_DEFAULT_XSL_SLIDESPDF', ['slides','fo','plain.xsl'])

    # Setup builder
    __builder = __select_builder(__lxml_builder, __libxml2_builder, __xsltproc_builder)

    # Create targets
    result = []
    for t,s in zip(target,source):
        t, stem = __ensure_suffix_stem(t, '.pdf')
        xsl = __builder.__call__(env, stem+'.fo', s, **kw)
        env.Depends(xsl, kw['DOCBOOK_XSL'])        
        result.extend(xsl)
        result.extend(__fop_builder.__call__(env, t, xsl, **kw))

    return result