Function: nffactorback
Section: number_fields
C-Name: nffactorback
Prototype: GGDG
Help: nffactorback(nf,f,{e}): given a factorisation f, returns
 the factored object back as an nf element.
Doc: gives back the \kbd{nf} element corresponding to a factorization.
 The integer $1$ corresponds to the empty factorization.

 If $e$ is present, $e$ and $f$ must be vectors of the same length ($e$ being
 integral), and the corresponding factorization is the product of the
 $f[i]^{e[i]}$.

 If not, and $f$ is vector, it is understood as in the preceding case with $e$
 a vector of 1s: we return the product of the $f[i]$. Finally, $f$ can be a
 regular factorization matrix.
 \bprog
 ? nf = nfinit(y^2+1);
 ? nffactorback(nf, [3, y+1, [1,2]~], [1, 2, 3])
 %2 = [12, -66]~
 ? 3 * (I+1)^2 * (1+2*I)^3
 %3 = 12 - 66*I
 @eprog