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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 get_concatenated_pdf_in_memory( pdf_plans: Iterable[PdfPlan], start_recto: bool = True) -> bytes: """ Concatenates PDFs and returns them as an in-memory binary PDF. Args: pdf_plans: iterable of :class:`PdfPlan` objects start_recto: start a new right-hand page for each new PDF? Returns: concatenated PDF, as ``bytes`` """

    writer = PdfFileWriter()
    for pdfplan in pdf_plans:
        pdfplan.add_to_writer(writer, start_recto=start_recto)
    return pdf_from_writer(writer)