computes the irreducible factors (as an ideal) of the polynomial
together with or without
the multiplicities (as an intvec) depending on the second argument:
0: returns factors and multiplicities, first factor is a constant.
May also be written with only one argument.
1: returns non-constant factors (no multiplicities).
2: returns non-constant factors and multiplicities.
Note:
Not implemented for the coefficient fields real and finite fields of
type (p^n,a).