[NTG-context] Minimal code for exporting custom tags

[Aditya Mahajan]

On Mon, 18 May 2020, Aditya Mahajan wrote:

>> Thanks! This also helped me find the documentation in hybrid.pdf.

Also epub-mkiv.pdf. Not sure why I didn't think of looking there.

> How do I pass options so that the export is
>
> <mycommand name=whatever>...</mycommand>
>
> (or some variant of that). I thought that
>
> \startelement[mycommand][name=whatever]
>
> should work (there are some such examples in the texmf tree), but it does 
> not.

epub-mkiv.pdf states that I need `\setupexport[properties=yes]`, but that doesn't work either:

```
\setupbackend[export=xml]
\setupexport [properties=my]

\setelementbackendtag[mycommand]
\setelementnature    [mycommand][mixed]

\starttext
\startsection[title={Sample document}]
   \startparagraph
     This is a paragraph
   \stopparagraph
   \startelement[mycommand][name=whatever]
     This is mycommand
   \stopelement
\stopsection
\stoptext
```

gives

```
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

<!--

     input filename   : test-tag
     processing date  : 2020-05-19T01:34:41-04:00
     context version  : 2020.05.09 15:37
     exporter version : 0.35

-->

<!-- This export file is used for filtering runtime only! -->

<document context="2020.05.09 15:37" date="2020-05-19T01:34:41-04:00" file="test-tag" language="en" version="0.35" xmlns:m="http://www.w3.org/1998/Math/MathML">
  <metadata>
  </metadata>
  <section detail="section" chain="section" implicit="1" level="3">
   <sectioncaption>
    <sectionnumber>1</sectionnumber>
    <sectiontitle>Sample document</sectiontitle>
   </sectioncaption>
   <sectioncontent>
    <paragraph>This is a paragraph</paragraph>
    <mycommand>This is mycommand</mycommand>
   </sectioncontent>
  </section>
</document>
```

What am I still missing?

On a different note, I didn't realize that interfaces.implement could create macros which could be used like this:

```
\unexpanded\def\dostartexport
   {%\glet\dostartexport\relax
    \let\currentexport\empty
    \clf_setupexport
        align      {\exportparameter\c!align}%
        bodyfont   \dimexpr\exportparameter\c!bodyfont\relax
        width      \dimexpr\exportparameter\c!width\relax
        properties {\exportparameter\c!properties}%
        hyphen     {\exportparameter\c!hyphen}%
        title      {\exportparameter\c!title}%
        subtitle   {\exportparameter\c!subtitle}%
        author     {\exportparameter\c!author}%
        firstpage  {\exportparameter\c!firstpage}%
        lastpage   {\exportparameter\c!lastpage}%
        svgstyle   {\exportparameter\c!svgstyle}%
        cssfile    {\exportparameter\c!cssfile}%
        file       {\exportparameter\c!file}%
        export     {\backendparameter\c!export}%
    \relax}

```

This nicely gets around TeX's limitation of 9 arguments!

Aditya