LE2 Registry: The types of links are in a (presumably IANA) registry, which can be extended using a well-understood process.
LE3 URIs: Non-centralized extensibility is achieved by using URIs to name types of links.
If people think it's a good idea, we could run a -1/0/+1 survey on the wiki (I'd be "0" on all four). If people want to speak up and argue that one of these options is substantially better than all the others, that would be appropriate: please include LE0, LE1, LE2, or LE3 in your subject line. If such an argument gets a lot of support and little opposition, we can discard a bunch of the Paces and move to focus on the syntax.
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