The notion of stack is one of the most important idea in computer science. The notion of recursive call of functions is usually taught in the first course of programming. I think it is important to understand how the stack is used to emulate recurisve calls. The idea is the use of the stack. Function arguments and local variables are stored in the stack. It enables the system to restore the values of the local variables and arguments after an execution of the function. However, it should be noted that, for each function call, the stack dynamically grows.

As an example that I used in a class room,
let us evaluate the -th Fibonacci number
defined by

by using a recurisive call.

/fib { /arg1 set [/n /ans] pushVariables pstack /n arg1 def (n=) messagen n message (-------------------------------) message n 2 le { /ans 1 def } { n 1 sub fib n 2 sub fib add /ans set } ifelse /arg1 ans def popVariables arg1 } defThe program would return the -th Fibonacci number. That is,

` 4 fib :: `

would return .
It also output the entire stack at each call,
so you can observe how stack grows during the computation
and how local variables