ufrac_auth: don't use curly braces for fractions since these are different
This is the one breaking change we planned in #412 for adjusting the existing notation. As discussed there, the ufrac_auth
fraction is somewhat special in that (a) on the ●
element, it is not splittable, and (b) on both elements, it is not bound by 1. We hence can't really make it a dfrac
(which is bound by 1), so to avoid confusion we should use a different notation.