/*
 *  stackframe structuur sf(n, a, b, returnvalue (rv), label)
 */

Stack S
push sf(n, ?, ?, ?, end)

call:
    if Top(S).n <= 1 then
        Top(S).rv = 1
        goto Top(S).label
    else
        push sf(Top(S).n - 1, ?, ?, ?, A)
        goto call
A:
    a = Top(S).rv
    Pop(S)
    Top(S).a = a
    push(Top(S).n - 2, ?, ?, ?, B)
    goto call
B:
    b = Top(S).rv
    pop(S)
    Top(S).b = b
    Top(S).rv = Top(S).a + Top(S).b
    goto Top(S).label
end:
    return Top(S).rv