It has been claimed in Section 3.2.4 that the object-oriented callback concept of the X Toolkit may be generalized in a very straightforward manner and successfully applied where a proper decoupling and high flexibility of the control flow is desirable. This generalization is applicable to the problem of subprocess control and provides a very proper solution.
Events coming from the X Window system are passed to the XLISP interpreter and will cause callback expressions being put into the callback queue, provided that a LISP expression was associated with the activated widget at creation time.
The very same callback concept is also used for the control of simulator execution. If a simulation tool terminates, it signals its termination to the parent process, which catches the signal and causes an associated callback expression to be put into the callback queue. The required callback expression must be passed to the LISP function that starts the subprocess.