find the actual source of the common "automatically ending `eb' diversion
on exit" error. This generally means you tried something like `.Em Do not'
which accidently calls the `Do' request, which _must_ be followed by Dc.
* Add a .Me request for marking up menu entries.
* Print the source line number on the "Extraneous .Ed" error. (Duhh.)
* New requests for HTML integration: .Mt (mailto) and .Lk (link).
* Rename the .em built-in to e@ and print an error if .em is ever
accidently invoked.
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