A CLIST can invoke another CLIST, which in turn can invoke another, and so forth. CLISTs that are invoked by other CLISTs are called nested CLISTs. When a nested CLIST ends, it automatically branches back to the statement following the one that invoked it. You can define global variables that allow nested CLISTs to communicate with each other.

You can structure a series of nested CLISTs in levels. The CLIST invoked by the user is the top-level or outer-level CLIST in the nesting chain. CLISTs invoked by the outer-level CLIST are nested within it, and they may have lower-level CLISTs nested within them.

In Figure 1, PROC1 is the outer-level CLIST. It invokes PROC2 and then PROC3, which are nested within it. PROC2 invokes PROC4, and PROC4 invokes PROC5. PROC4 is nested within PROC2, and PROC5 within PROC4.

Figure 1. Nested CLISTs

Because CLISTs are executed sequentially, PROC1 cannot invoke PROC3 until PROC5, PROC4, and PROC2 finish processing.

The same CLIST can be invoked at two or more levels of a nested hierarchy because each invocation of a nested CLIST causes a new copy of it to be brought into storage. For example, PROC2 and PROC4 can both invoke PROC5.