Controls when objects can be promoted to the next generation.
The generation number affected.
The number of garbage collections survived by objects in that generation, before promotion. If count is
, the function returns the current promotion count setting.
An integer specifying which segment of the generation is to be affected. The default is 0, meaning the lowest segment of the generation.
Controls how many garbage collections an object in a segment must survive before promotion to the next generation.
LispWorks Reference Manual - 6 Apr 2005