I am looking into fixing a bunch of complicated and slightly erroneous logic that is caused by subtree addresses being optional, and it occurred to me that at the same time, I should address some longstanding issues with hyperlinks.

Right now, links are formed using Markdown-style syntax [title](uri) or [[uri]], and uri can be either a local tree address (like xxx-NNNN) or it can be an ordinary web address.

I have found this to be problematic because it is impossible to give useful warnings for broken links, because we certainly cannot easily detect all possible web addresses. I think the right solution is to have a different syntax for actual external links, and only use the Markdown-style links for the internal links to actual trees.