f.x := 1;
f.y := null;
A Field Sensitive analysis may be able to tell that f.y is null, and
f.x is non-null, while a Field Insensitive analysis will consider
f.x and f.y to be equivalent and not be able to conclude whether
f.☐ does or does not point to null. (Here, we use ☐ as a
wildcard for "any field").