Function pick_op 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pick_op(
    stack: *mut StackValue,
) -> *mut StackValue

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

Safety

Stack must have at least n+1 values (plus the index value).

Errors

Sets runtime error if:

• The top value is not an Int
• n is negative
• n exceeds the current stack depth