to return to the top-level of ACL2's command loop

When (a!) is evaluated inside of ACL2's command loop, the current computation is aborted and control returns to the top of the command loop, exactly as though the user had interrupted and aborted the current computation. (Note: Versions of ACL2 up to Version_3.4 provided `#.' for this purpose, but no longer; see sharp-dot-reader.)

If you are at an ACL2 prompt (as opposed to a raw Lisp break), then you may type :a! in place of (a!); see keyword-commands.

For a related feature that only pops up one level, see p!.

Logically speaking, (a!) = nil. But imagine that it is defined in such a way that it causes a stack overflow or other resource exhaustion when called.