How to work with common fractions?
2_3+7_9 results in 13_9, but not 1_4_9.

Also I can't enter 1_4_9 in an expression, but only 13_9

Please use standard math notation, otherwise you'll make quite obvious errors.
I guess that with _ you intend a division, hence /,

2/3+7/9=13/9

13/9 is also 1+4/9, not 1*4/9 or 1/4/9.

The problem is not SMath...