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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_page_vert_percentile( mention, page_width=DEFAULT_WIDTH, page_height=DEFAULT_HEIGHT ):
 """Return which percentile from the TOP in the page the Mention is located in. Percentile is calculated where the top of the page is 0.0, and the bottom of the page is 1.0. For example, a Mention in at the top 1/4 of the page will have a percentile of 0.25. Page width and height are based on pt values:: Letter 612x792 Tabloid 792x1224 Ledger 1224x792 Legal 612x1008 Statement 396x612 Executive 540x720 A0 2384x3371 A1 1685x2384 A2 1190x1684 A3 842x1190 A4 595x842 A4Small 595x842 A5 420x595 B4 729x1032 B5 516x729 Folio 612x936 Quarto 610x780 10x14 720x1008 and should match the source documents. Letter size is used by default. Note that if a candidate is passed in, only the vertical percentil of its first Mention is returned. :param mention: The Mention to evaluate :param page_width: The width of the page. Default to Letter paper width. :param page_height: The heigh of the page. Default to Letter paper height. :rtype: float in [0.0, 1.0] """

    span = _to_span(mention)
    return bbox_from_span(span).top / page_height