[Groff] explicit and implicit spacing in eqn

[Doug McIlroy]

I may have made this comment before. My apologies if
it's already on someone's to-do list.

Groff eqn, unlike the original Unix program, attempts to
vary spacing according to mathematical custom. Thus it
automatically adds apace around the equal sign in
        a=b
while in the old days one had to put the space in
manually
        a~=~b
Somewhat surprisingly, groff eqn, adds space around
Unknown tilde escape.
= even in the presence of ~, seemingly as if ~ were
treated as a letter. This is relatively unnoticeable.
But it is not unnoticeable in case of unary minus.
Groff eqn treats
        a~=~-b
as if it were binary, and inserts extra space between
the - and b.

I have a lot of eqn that this interpretation damages.
If the problem is something as simple as treating ~
as a letter, it should be easy to fix.

Doug