> -- Again, if there was a better parser at hand, one would not need to have
 > any markup at all, because it would be able to see that `20' is the object

give up, unless you want to wait for a parser which "understands" human
language (or write one but don't hand wave it)

how do you do without markup in this case:

  The $a$ in the formula is a variable

while contrieved i came across that particular problem in math when i tried to
understand an article in Hungarian (i think) about number theory and misstook
an "a" being text as part of a longer inline formula because it was
incorrectly coded (by you perhaps?) ie not identifiable easily as math not
text.

frank