How does substraction happen in MIXAL?
 

I am currently reading the Art of Computer Programming by Donald Knuth, volume 1, 3rd edition and cannot understand the way the substraction operator works in MIXAL(specifically example 2 from page 132). SUB 1000 [-][1234][0][0][9] rA before [-][2000][150][0] Cell 1000 [+][ 766][149][?] rA...