Traditionally, programs are built on the concept of a call stack. This concept is pretty straightforward because a program is basically a path of execution and invocation of sequential operations. Every operation can invoke another operation. At the time of invocation, a program creates a context for the callee operation. The caller operation will wait for the callee operation to return and the program will restore the context of it. Finally, the caller continues with their next operation. The callee operation might have executed another operation on its own behalf.

The program creates a call stack to coordinate and manage the context of each call. The basic primitives of this concept are calls. All calls in the program are tightly coupled, because the program knows which operation must...