Consecutive keys in a binary search tree

Two stored keys $a<b$ are consecutive when no stored key lies strictly between them. Their numerical difference need not be one. After splaying on $a$ followed by splaying on $b$, the node $a$ is the left child of $b$ with an empty right subtree. To prove this, after the first call $b$ is the minimum of the right subtree of $a$. The last step crossing $a$ is either a zig step or a right-left zig-zag step; both make $a$ the left child of $b$. A nonempty right subtree of $a$ would require a stored key strictly between them.