Rebuild a fraction from it's value modulo p : fracmod

fracmod takes two arguments, an integer n (representing a fraction) and an integer p (the modulus).
If possible, fracmod returns a fraction a/b such that

- $${\frac{{\sqrt{p}}}{{2}}}$$ < a $$\leq$$ $${\frac{{\sqrt{p}}}{{2}}}$$,    0 $$\leq$$ b < $${\frac{{\sqrt{p}}}{{2}}}$$,    n×b = a(mod p)

In other words n = a/b(mod p).
Input :

fracmod(3,13)

Output :

-1/4

Indeed : 3* -4 = - 12 = 1 ( mod 13), hence 3 = - 1/4%13.
Input :

fracmod(13,121)

Output :

-4/9

Indeed : 13× -9 = - 117 = 4 ( mod 121) hence 13 = - 4/9%13.