RESET-KILL-RING

save memory by resetting and perhaps resizing the kill ring used by oops 
Major Section:  HISTORY

By default, ACL2 holds on to old logical worlds when you undo commands (see ubt), as documented elswhere; see oops. You can free up memory by deleting those old worlds using reset-kill-ring. 

Examples:
(reset-kill-ring t state)   ; replace each element of the kill ring by nil
(reset-kill-ring 2 state)   ; create a new kill ring of '(nil nil)
(reset-kill-ring 0 state)   ; create a new kill ring that is empty
(reset-kill-ring nil state) ; just return the length of the kill ring


General form:
(reset-kill-ring n state)
where n evaluates either to t, to nil, or to a nonnegative integer (a natp). If n evaluates to t, it is treated as the length of the current kill ring. If n is nil, then the length k of the current kill ring is returned as a value triple (mv nil k state). If n is a natp, then the kill ring is replaced with a list of n nils.

In particular, use (reset-kill-ring 0 state) to avoid saving any old logical worlds, at the cost of disabling the effect of the oops command.