Support physical units about asciimathml HOT 2 OPEN

Suggesting something even simpler, e.g.

g = 9.81.m/s^2

Noting that dot directly following a number has no useful meaning (prints currently simply as a dot). Using some character other than dot could be safer, if it needs to be a strictly nonbreaking change. Either way, quotes at both ends of the unit are still quite ugly.

Units formatting also should add a narrow non-breaking space between the number and its unit where I believe \, is normally used in LaTeX for this.

Sometimes units need to be printed without a number so that is also a concern. Technically the dot prefix could also work for this:

[a] = .m/s^2

One concern is when to break out of unit mode, and in that regard the surrounding quotes are simpler. Otherwise spaces, parenthesis and division could either be part of the unit expression or a continuation of the equation (after which letters should appear as variables again).

from asciimathml.

There is an old note from 2003 on how to use units in MathML https://www.w3.org/TR/mathml-units/ with a short segment on presentation MathML.

<math xmlns="http://www.w3.org/1998/Math/MathML">
<mstyle>
<mn>42</mn>
<mo rspace="thickmathspace">&InvisibleTimes;</mo>
<mi mathvariant="normal" class="MathML-Unit">N</mi>
<mo>=</mo>
<mn>42</mn>
<mo rspace="thickmathspace">&InvisibleTimes;</mo>
<mi mathvariant="normal" class="MathML-Unit">kg</mi>
<mo>&middot;</mo>
<mfrac>
<mi mathvariant="normal" class="MathML-Unit">m</mi>
<msup>
<mi mathvariant="normal" class="MathML-Unit">s</mi>
<mn>2</mn>
</msup>
</mfrac>
</mstyle>
</math>

One could maybe write the si units in full

42*newton = 42*kilogram*metre/second^2

a bit similar to https://ctan.org/pkg/siunitx?

from asciimathml.