There is a manual about math but it's not complete

If you mean math-mkiv, it doesn't shed lights about fractions. I'm seeing it's more illustrating about subjects as integrals and other stuff, tho.
Reading the mailing list, I realized there's a simplecommand key for matrices, among other nice things which maybe should be... if not documented, at least mentioned. 
 
we have to protect our reputation of not
having manuals

lmao

But ... contributions (examples etc, as I can blabla
the rest) are welcome.
 
I'll take note of that. Thank you very much, Hans.

Jairo :)