Function pick 

pub unsafe fn pick(stack: Stack, n: usize) -> Stack

Pick: Copy the nth value to the top (0-indexed from top) ( … xn … x1 x0 n – … xn … x1 x0 xn )

Examples:

• pick(0) is equivalent to dup
• pick(1) is equivalent to over
• pick(2) copies the third value to the top

Safety

Stack must have at least n+1 values