that and ^ do the same thing (but there's also ^^, ^^^).
thatexpr is like CL + .
But (%) is like (eval thatexpr), except % is a macro that gives you what the last thing entered expanded into. And so, since % is a macro, it itself is expanded away. Hence
arc> 2
2
arc> (* ^ 2)
4
arc> (%) ; equivalent to entering (* ^ 2) again
8
arc> (%) ; and again
16
arc> (%) ; and again
32
Whereas
arc> 2
2
arc> (* ^ 2)
4
arc> (eval thatexpr) ; i.e. (eval (* ^ 2))
8
arc> (eval thatexpr) ; i.e. (eval (eval thatexpr)) i.e. (eval (eval (eval thatexpr))) etc.
[infinite loop]
