intersection

Ideal intersections in the ring of differential operators.
Example 1: [[[(x1) (x2)] [(x2) (x4)] (x1,x2,x3,x4)] intersection
             [(x2) (x4^2)] (x1,x2,x3,x4)] intersection :: 
Example 2: [[[(x1) (x2)] [(x2) (x4)] (x1,x2,x3,x4)] intersection
             [(x2) (x4^2)] (x1,x2,x3,x4)] intersection /ff set ff message
           [ ff [(x2^2) (x3) (x4)] (x1,x2,x3,x4)] intersection :: 
Example 3: [[[(x1) (x2)] [(x2) (x4^2)] (x1,x2,x3,x4)] intersection
             [(x2^2) (x3) (x4)] (x1,x2,x3,x4)] intersection ::