Understanding the call stack operation

The task requires analyzing how the call stack is formed and removed during sequential function calls. You need to understand which variables are in the stack at each moment of program execution.

func a3(){
    let x = 5
}
func a1(){
    let y = 5
    a2()
}
func a2(){
    let z = 3
    a3()
}
a1()

Determine how the stack changes from the start of a1() execution to the complete termination of all functions.