We will say for short that an entire node u in τ v matches a node half h if it is clear from the context which half of u matches h. Not all correspondences are useful when performing a verification.
we must require that the region of τ v that matches node halves in τ is contiguous and starts at the root of τ v ; and if some node in τ v has unmatched children, then these must be the rightmost children of their parent.