code: def build_markdown(toc_headlines, body, spacer=0, placeholder=None):
      
      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
docs: 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