A class full of students.
An empty classroom.
The class is either full or empty.
exists s1 x1 x2 x3.(a_topic(x1) & of(s1,x3) & n_student(x3) & Theme(s1,x2) & a_full(s1) & (x1 = x2) & n_class(x1))
exists s1 x1.(a_topic(x1) & n_classroom(x1) & a_empty(s1) & Theme(s1,x1))