There is a large enough array $A$ with indices $p$ and $q$ which are initialized by 0, and functions $f(x)$ and $g()$ are defined as follows. Array $A$ and indices $p$ and $q$ can only be accessed in the functions $f(x)$ and $g()$.

$f(x)$:

• $p \leftarrow p+1$
• $A[p] \leftarrow x$

$g()$:

• $x \leftarrow A[p]$
• $p \leftarrow p-1$
• return $x$