JOIN

• Inputs: T1, T2, and D.
• Outputs: T1' and T2'.
• Preconditions: D is not `up', T2 is not a subtree, T2 is not empty, and T1's subtree in direction D does not exist.
• Postconditions: if T1 is empty, T1' is unified with T2. If T1 is nonempty, T1' has T2' as its subtree in direction D.

JOIN(T1,T2,Left) produces: