How do I make a \therefore in *plain tex* ?

[Peter King]

On 4/30/23 18:30, Steve Litt wrote:
> Does anybody know how to make the \therefore symbol from the amssym
>>> package grow and shrink with the math font?
>> \input amssym
>>
>> $ \therefore $
>>
>> \font \tenmsa = msam10 scaled \magstep 2
> [snip remainder of source code]
>
> Thanks Philip,
>
> I'll try it tomorrow and report back.
>
> SteveT

I took an entirely different approach to this problem years ago, by 
constructing a simple "therefore" in hollow circles out of a few 
primitives in plain TeX:

%% Hollowbox ``therefore''
\def\THF{\leavevmode\lower0.2ex\hbox{$\circ$}\kern-0.2em\raise0.7ex\hbox{$\circ$}\kern-0.2em\lower0.2ex\hbox{$\circ$}\thinspace}

You can replace the $\circ$ with a bullet to get a filled-in 
"therefore".  No fuss with alternate fonts or loading other macros.  
Your mileage may vary, of course.

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