[qdog]

Maybe 'register' wasn't abstract enough, he means something like (in psuedo-assembly):

  ldr r1,4     ;Load constant 4 into register 1
  add r2,r2,r1 ;Add contents of r2 and r1, store in r2
  ldr r1,5     ;Load constant 5 into register 1

where the value is only temporarily available for the single calculation. Even without thinking of registers, you can refer to the loop variable, but you can't refer to numbers in your operations.

Maybe he could update this line for clarity, though.