(6) holds because a common subtree by extending from ( 1 , 2 ) can be formed by taking the hyper-edge ( 1 , 2 ), together with a choice at each of their leaf nodes of simply taking the non-terminal at the leaf node, or any one of the common subtrees with root at the leaf node.
there are 1 + Δ ′ 1 , , 2 , possible choices at the j th leaf node.