Right, the reason it won't ever emit F is because RND(1) will never return 1, it's implementation will return 0<=X<1, and INT() is a truncation, so INT(RND(1) + 15) will return an integer between 0 and 14. Adding one gives 1 to 15, and since F is in the 16th position of the string (Apple BASIC indexed from 1, not 0) it will never get selected.