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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 build_markdown(toc_headlines, body, spacer=0, placeholder=None):
 """ Returns a string with the Markdown output contents incl. the table of contents. Keyword arguments: toc_headlines: lines for the table of contents as created by the create_toc function. body: contents of the Markdown file including ID-anchor tags as returned by the tag_and_collect function. spacer: Adds vertical space after the table of contents. Height in pixels. placeholder: If a placeholder string is provided, the placeholder will be replaced by the TOC instead of inserting the TOC at the top of the document """

    if spacer:
        spacer_line = ['\n<div style="height:%spx;"></div>\n' % (spacer)]
        toc_markdown = "\n".join(toc_headlines + spacer_line)
    else:
        toc_markdown = "\n".join(toc_headlines)

    body_markdown = "\n".join(body).strip()

    if placeholder:
        markdown = body_markdown.replace(placeholder, toc_markdown)
    else:
        markdown = toc_markdown + body_markdown

    return markdown