Demos

This is a formula with a big summation and the number pi $$\sum _{n=0}^{+\mathrm{\infty }}\frac{1}{n^2}=\frac{{\pi }^2}{6}$$ This is a test $$\underset{27}{\overset{\mathrm{\infty }}{cat}}=<>=\underset{27}{\overset{\mathrm{\infty }}{cat}}$$ An annotated test case.
The shiny new approach is to generate a list of valid caret positions. $\frac{\sqrt{\mathrm{<!--|-->\pi <!--|-->}}}{\mathrm{<!--|-->6<!--|-->}}+\frac{\mathrm{<!--|-->\pi <!--|-->}}{\mathrm{<!--|-->6<!--|-->}}+\sqrt{\sqrt{\mathrm{<!--|-->5<!--|-->}}}++\underset{\mathrm{<!--|-->2<!--|-->7<!--|-->}}{\overset{\mathrm{<!--|-->\infty <!--|-->}}{<!--|-->c<!--|-->}}$ And a quick msup torture test
$-2^3+2^{4^8}*{2^3}^3$
And nested mrows are also fun
$-2$
Mroot $\sqrt[3]{\sqrt{\frac{1}{2}+4}}+0$ Sqrt test. See HTML comments for preferred order $\sqrt[\mathrm{<!--2\; (should\; be\; outside\; of\; mn)-->\; a\; <!--3\; (should\; be\; outside\; of\; mn)-->}]{\sqrt{\mathrm{b}}}$