How to use the panflute.Plain function in panflute

def finalize(doc):
	c1 = pf.TableCell(pf.Plain(pf.Str("Element")))
	c2 = pf.TableCell(pf.Plain(pf.Str("Frequency")))
	header = pf.TableRow(c1, c2)
	rows = []
	for tag in doc.counter:
		c1 = pf.TableCell(pf.Plain(pf.Str(tag)))
		c2 = pf.TableCell(pf.Plain(pf.Str(str(doc.counter[tag]))))
		rows.append(pf.TableRow(c1, c2))
	table = pf.Table(*rows, header=header)
	doc.content = [table] # bugbug?

if backmatter:
            doc.backmatter.append(ans)
            msg = '\hyperref[fig:{}]{{[\Cref{{fig:{}}} Goes Here]}}'
            msg = msg.format(label, label)
            return pf.Plain(pf.Str(msg))
        else:
            return ans
    else:
        title = pf.convert_markdown(title)
        assert len(title)==1, title
        title = (title[0]).items

        notes = pf.Div(*pf.convert_markdown(notes), classes=['note'])
        title_text = pf.stringify(title)
        img = pf.Image(*title, url=fn, title=title_text, identifier=label)
        ans = pf.Div(pf.Plain(img), pf.Plain(pf.LineBreak), notes, classes=['figure'])
        return ans

def to_latex(elem):
    """
    Convert element to LaTeX.

    Arguments
    ---------
        elem: elem to convert

    Returns
    -------
        LaTex string
    """
    return convert_text(
        run_filters([remove_useless_latex], doc=Plain(elem)),
        input_format="panflute",
        output_format="latex",
        extra_args=["--no-highlight"],
    )

pagebreak = doc.get_metadata('format.media-pagebreak', False)

        w = options.get('width', 1.0)
        subs['width'] = w
        subs['innerwidth'] = options.get('innerwidth', w) / w
        subs['notesize'] = options.get('notesize', 'small')
        subs['pagebreak'] = '\\clearpage\n' if pagebreak else ''

        text = LATEX_TEMPLATE.safe_substitute(subs)
        ans = pf.RawBlock(text=text, format='latex')

        if backmatter:
            doc.backmatter.append(ans)
            msg = '\hyperref[fig:{}]{{[\Cref{{fig:{}}} Goes Here]}}'
            msg = msg.format(label, label)
            return pf.Plain(pf.Str(msg))
        else:
            return ans
    else:
        title = pf.convert_markdown(title)
        assert len(title)==1, title
        title = (title[0]).items

        notes = pf.Div(*pf.convert_markdown(notes), classes=['note'])
        title_text = pf.stringify(title)
        img = pf.Image(*title, url=fn, title=title_text, identifier=label)
        ans = pf.Div(pf.Plain(img), pf.Plain(pf.LineBreak), notes, classes=['figure'])
        return ans

def plain_to_table_cell(string):
        return panflute.TableCell(panflute.Plain(panflute.Str(string)))

pagebreak = doc.get_metadata('format.media-pagebreak', False)

        w = options.get('width', 1.0)
        subs['width'] = w
        subs['innerwidth'] = options.get('innerwidth', w) / w
        subs['notesize'] = options.get('notesize', 'small')
        subs['pagebreak'] = '\\clearpage\n' if pagebreak else ''

        text = LATEX_TEMPLATE.safe_substitute(subs)
        ans = pf.RawBlock(text=text, format='latex')

        if backmatter:
            doc.backmatter.append(ans)
            msg = '\hyperref[fig:{}]{{[\Cref{{fig:{}}} Goes Here]}}'
            msg = msg.format(label, label)
            return pf.Plain(pf.Str(msg))
        else:
            return ans
    else:
        title = pf.convert_markdown(title)
        assert len(title)==1, title
        title = (title[0]).items

        notes = pf.Div(*pf.convert_markdown(notes), classes=['note'])
        title_text = pf.stringify(title)
        img = pf.Image(*title, url=fn, title=title_text, identifier=label)
        ans = pf.Div(pf.Plain(img), pf.Plain(pf.LineBreak), notes, classes=['figure'])
        return ans

def action(elem, doc):
    if isinstance(elem, pf.Doc):
        version = pkg_resources.get_distribution("panflute").version
        json_serializer = lambda elem: elem.to_json()
        raw = json.dumps(elem, default=json_serializer)
        raw = json.loads(raw)
        raw = json.dumps(raw, check_circular=False,
                         indent=4, separators=(',', ': '))
        disclaimer = pf.Para(pf.Emph(pf.Str('Note: sort order not preserved')))
        elem.content = [
          pf.Header(pf.Str('Python version:'), level=2),
          pf.Para(pf.Str(sys.version)),
          pf.Header(pf.Str('Panflute version:'), level=2),
          pf.Para(pf.Str(version)),
          pf.Header(pf.Str('sys.argv:'), level=2),
          pf.Plain(pf.Str(str(sys.argv))),
          pf.Header(pf.Str('JSON Input:'), level=2),
          disclaimer,
          pf.CodeBlock(raw)
        ]

Compute other code for table.

    Arguments
    ---------
        doc: pandoc document
        category: category numbered
        definition: definition
    """
    if category in doc.collections:
        # Prepare the list
        elements = []
        # Loop on the collection
        for tag in doc.collections[category]:
            # Add an item to the list
            elements.append(
                ListItem(Plain(Link(doc.information[tag].entry, url="#" + tag)))
            )
        # Return a bullet list
        return BulletList(*elements)
    return None