Atomically exchange a place value with a new value, returning the old value.
atomic-exchange place new-value => old-value
One of the specific set of places defined for low level atomic operations.
atomic-exchange exchanges the value in place with new-value, returning the old-value. The operation is guaranteed to be atomic.
place must be one of the places described in Low level atomic operations, or expand to one of them.
LispWorks User Guide and Reference Manual - 13 Feb 2015