6. (4%) A binary search tree can be augmented for specialized applications such as an order statistic tree. The provided pseudo-code is an implementation of the OS-SELECT function, which retrieves the $i$-th smallest element in the set represented by the binary search tree. The variable $x$ represents a node in the tree and initially points to the root when the function is called. As the function recurses, $x$ updates to reflect the current node being processed. Your task is to fill in the missing part of the pseudo-code in line 1.

OS-SELECT(x, i)
1   r ← ______
2   if i = r
3       then return x
4   else if i < r
5       then return OS-SELECT(left[x], i)
6   else return OS-SELECT(right[x], i - r)