Maintain Credit Rule: Zig-Zig Case

We're splaying x, with y and z as the immediate parents.

The function W gives the subtree sizes before the pair of rotations, W' gives the sizes after.

• We owe no more than credits before the rotation pair.
• Pay off the debt using the credits from x, y, and z.
• Restructure the tree.
• Replace the credits needed on x, y, and z by taking out a loan of .

Same idea works for Zig-Zag and Zig.