3. course numbers of never tought courses

RO = project grades on cno
cno
cs110
cs110
cs110
cs110
cs110
cs110
cs240
cs240
cs240
cs410
cs410
cs210
cs210
cs210
cs210
cs210
cs240
cs240
cs240
cs310
cs310
R1 = project courses on cno
cno
cs110
cs210
cs240
cs310
cs350
cs410
cs460
R2 = difference of R1 to R0
cno
cs460
cs350