On 12/23/2015 3:19 AM, Alan BRASLAU wrote:

So it it a TeX programming limitation.

The risk of leading to an exponential number of branches is addressed
by Knuth at the beginning of Chapter 17 of the TeXbook (p. 139):
"Mathematicians tend to \quote{overuse} \over when they first begin to
typeset their own work on a system like \TEX." ...

At one point, I went back to using \over rather than \frac as the later
was broken and gave no output. This has since been fixed, but I stayed
with \over as I find primary operators to be much more elegant syntax
than multiple argument macros. It is too bad that luatex does not
employ a better programming solution, as the use of \frac{}{} leads to
ugly mathematical source code.

Well, it is not impossible in mkiv do make something 1\alansover2 that adjusts itself to some settings (i could probably do it right now) but who will use it (apart from you) and who would document it then ..