-ms anomaly

[Doug McIlroy]

>> Nevertheless, -ms is misinterpreting the output of eqn in this
>> case.
> 
> It appears to be intentional. At least in groff's implementation
> of the ms macros, the first call to a paragraph macro is special,
> in that it terminates the front matter and begins the body text,
> and redefines a number of macros in the process, including EQ/EN.
> (Setting off the equation on a line of its own is the job of the
> EQ/EN macros, not of eqn.)

I suspect it's accidental. Further experiment shows that .EQ/.EN
work properly in the abstract (.AB/.AE) but not in the title (.TL).
This may be an artifact of limited memory on the PDP-11. A lot of
the original -ms was concerned with front matter, including the
details of cover sheets for Bell Labs internal memoranda. To
squeeze the package size, the set of available macros changed
after front matter was complete. Groff appears to perpetuate
that hack.

Regardless of how the behavior arose, it is certainly a bug for
-ms not to implement the semantics intended by eqn.

In my experimentation, I came on a related quirk. .AB and .AE
are noisily rejected and ignored if not preceded by .TL.
This behavior is rather presumptuous. Almost all groff runs
are drafts. Why must an incomplete paper have a title in
order to be typeset correctly?

I hit the .EQ/EN anomaly precisely because I was dealing with
a fragment of a paper. I was debugging some hairy equations 
that were to be slipped into a long manuscript later.

Doug