Function: factor
Section: number_theoretical
C-Name: factor0
Prototype: GD-1,L,
Help: factor(x,{lim}): factorization of x. lim is optional and can be set
 whenever x is of (possibly recursive) rational type. If lim is set return
 partial factorization, using primes up to lim (up to primelimit if lim=0)
Description: 
 (int, ?-1):gen        Z_factor($1)
 (gen, ?-1):gen        factor($1)
 (int, 0):gen          smallfact($1)
 (gen, small):gen      factor0($1, $2)

