Requests to activate load module and program object libraries into the search chain can be stacked. You control this with the COND and UNCOND operands of the TSOLIB command.

By default, a request to activate libraries is done unconditionally. The request becomes the current and active one. Any previous request (if one was issued) is stacked and temporarily made inactive. The next request to deactivate libraries will remove the current and active one from the search chain and re-activate the previous request (if there was one).

If a request is made conditionally, by using the COND keyword operand, the request will not become active if any previous activation took place before. See TSOLIB command examples for a detailed example on how to stack load module library requests.