Rational expressions

If a division by 0 occurs, i.e. $S_n(p/q) = S_n(p) / S_n(q) \bmod{n}$and S_n(q)=0, then we try to prove (recursively) that q is identically zero. If it is true, then the original expression does not make mathematical sense. If we have proof that the denominator is not zero, this means that we have found an n for which $S_n(q) \neq 0$, and hence can continue computing the signature of the original expression with this n. If the division fails because S_n(q) | n, when n is not prime, then we can try with different values of n. In some cases, when there are other restrictions on n, it may not be possible to compute the division. For example, if n is restricted to be a multiple of 2, then the expression $S_n ( \frac{1}{x(x+1)} )$ cannot be computed.