A placeholder on Gemini replies

Partly in response to Solderpunk and in continuing dialogue with Shufei.

I'm a bit overextended right now... was busy over the weekend with garden, Friends Meeting, children, and dealing with the political situation in the US. Stayed up too late last night following news and had to nap today. Plus it's Monday and I'm back at work (from home). I want to keep my foot in the Gemini replies discourse, but I don't have the time to write a full post.

I also really *need* to write a Send The Nukes[1] post, but I'm a little demoralized even for that...

[1] President Xi, Send The Nukes!

Anyway, I'm in agreement with Solderpunk on basically how reply notifications work:

Find the pingback endpoint for the page.
Post the URL of your reply to the endpoint.
The endpoint script/program checks that it's been given a URL, and that the URL returns text/gemini content, and that said content contains a link back to the original post. Otherwise, it discards it.
The endpoint does *something*: it may notify the OP by mail, add to a feed, add to a static or dynamic page, etc.

Solderpunk proposes a well-known URL, such as gemini://my.site/.well-known/pingback (my example, not his), and wonders how this will work with multiuser sites.

My current thinking is:

A page is pingback-eligible if the last line of the page is a link, and that link contains a marker that I haven't figured out yet. (?rel=reply in the query string? Some other convention?)
The reply endpoint is that link.
If it is a gemini:// link, then follow the protocol above.
If it is a mailto: link, then send email appropriately, including the link to the reply.

This solves the problem of finding an endpoint on a multiuser site, and it provides a solution for completely static sites with no (S)CGI capabilities: use a mailto: link. I envision replies being made by a stand-alone command-line tool, or built into common clients, which may hand off the mail part to the user's default email program.