monomialSubideal -- find the largest monomial ideal in an ideal

i1 : QQ[a,b,c,d];

i2 : I = ideal(b*c, c^2 - b*d, -a*c+b^2)

                  2         2
o2 = ideal (b*c, c  - b*d, b  - a*c)

o2 : Ideal of QQ [a, b, c, d]

i3 : monomialSubideal I

                     3          2   3          2
o3 = monomialIdeal (b , b*c, a*c , c , a*b*d, b d, a*c*d)

o3 : MonomialIdeal of QQ [a, b, c, d]