A way for a template (e.g. single.html) to signal a flow (or pass data) in the base template

[bep]

See A way for a template (e.g. single.html) to signal a flow in the base template · Issue #11023 · gohugoio/hugo · GitHub

I’m not sure I managed to describe the problem it solves well, but I find it very useful, so I thought I’d share.