pub fn peek_prev(&self) -> Option<&'a T> {
        unsafe {
            let prev = match self.current {
                None => self.list.tail,
                Some(current) => current.as_ref().prev,
            };
            prev.map(|prev| &(*prev.as_ptr()).element)
        }
    }
Returns a reference to the element that the cursor is currently
pointing to.

This returns `None` if the cursor is currently pointing to the
"ghost" non-element.
benchmarks/haystacks/code/rust-library.rs