* * %$\jacobi{a}{-1} = 1$% if @a@ is negative, or 1 if
* positive;
* * %$\jacobi{a}{0} = 0$%;
* * %$\jacobi{a}{-1} = 1$% if @a@ is negative, or 1 if
* positive;
* * %$\jacobi{a}{0} = 0$%;
* congruent to 1 or 7 (mod 8), or %$-1$% otherwise.
*
* If %$n$% is positive and odd, then this is the Jacobi
* congruent to 1 or 7 (mod 8), or %$-1$% otherwise.
*
* If %$n$% is positive and odd, then this is the Jacobi