A place containing a COM interface pointer or
A COM interface pointer or
setf expander increments the reference count, as if by add-ref, of new-value, unless it is
nil. It then decrements the reference count, as if by release, of the existing value in iptr, unless this is
nil. Note that this order is important in the case that the new value is the same as the current value. Finally the value of place iptr is set to new-value.
LispWorks COM/Automation User Guide and Reference Manual - 14 Feb 2015