Function: factorback
Section: number_theoretical
C-Name: factorback0
Prototype: GDGDG
Help: factorback(f,{e},{nf}): given a factorisation f, gives the factored
 object back. If this is a prime ideal factorisation you must supply the
 corresponding number field as last argument. If e is present, f has to be a
 vector of the same length, and we return the product of the f[i]^e[i]

