SE547: Sequential noninterference [4/23]

Informal idea behind noninterference (make this formal later):

A program P satisfies noninterference if the contents of low security variables after running P is independent of the contents of high security variables before running P

Which of the following satisfy noninterference?

1. x_H=y_L;
2. y_L=x_H;
3. x_H=0; if(even(y_L)) { x_H=1; }
4. y_L=0; if(even(x_H)) { y_L=1; }