Which reminded me: can we change the last sentence to:
Unless explicitly allowed by a Sieve extension, the user can not
specify a namespace when setting variables with SET.
Once again, I don't like to be too restrictive for no good reason.
I'd prefer other extension to add their own action if they need it. the
way I figure is that most uses of namespaces will be read-only, or with
specialised needs for setting values. you don't really want
SET "mimestructure.body.2.1" "Hi there";
I would also prefer extensions to explicitly allow specifying a namespace. And
as Kjetil points out I think it very unlikely that we'd agree to any extension
that permitted setting of "silly" namespaces, but I think it likely that we'll
use namespaces a lot in the future of the language, and prohibiting the
possibility of setting variables in namespaces sounds to me like something we'd
regret. You said yourself "most" uses will be read-only. You didn't say "all"
uses will be read only.