create () is a mock domain manager.

When asked to run a new Eio domain, it just runs it in the parent domain. It runs the function in a context where id is a fresh domain ID (assigned sequentially starting from 1).

run fn runs fn dm, where dm is a new fake domain manager. It also runs with_domain_tracing to display domain IDs in trace output.

fn itself runs with id set to "0".

id is used to get or set the current fake domain's ID.

This is used in traceln output.

with_domain_tracing fn runs fn () with a modified traceln function that prefixes the current id (if any) to each trace message.