there is also #clone, which seems faster
on my computer:
fact _ [:n | n isZero ifTrue: [1] ifFalse: [n * (fact copy value: (n - 1))]].
[fact copy value: 6] bench ==> '157928.2143571286 per second.'
fact _ [:n | n isZero ifTrue: [1] ifFalse: [n * (fact clone value: (n - 1))]].
[fact clone value: 6] bench ==> '208975.4049190162 per second.'
Stef