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Let me take a step back. I think that may bring clarity.
Notations are a name-value pair in a signature, with arbitrary binary
data allowed as the name and value. (I would use the word "key" as
that's more traditional in data structures, but that would be
confusing.) If you allow arbitrary values for names, there's the
possibility of collisions in names. Hence a registry is desirable.
This registry is presently null.
We also allowed for an unmanaged space of the form foo(_at_)example(_dot_)com,
for the owners of any example.com domain, and if they step on their
own feet that's their problem.
There is also metadata. The only metadata we have is the single flag.
Beyond that flag, no one has thought of any other needed metadata,
and even that one flag is something we seem to have consensus on, and
that consensus is that it is more charming than useful. Thus, it
seems we need a registry for those flags because if we don't, then a
week from now someone will come up with a brilliant idea that needs
one. Murphy's Law is like that.
So there are two possible registries: one for the managed space of
names, and one for extra metadata flags. Does this clear it up?
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