Because the head words for the internal nodes are not marked in the PS, there are several possibilities in choosing the head words for the internal nodes: the head word of the Y can be a or b, and the head word of X can be c or the head word of Y , resulting in four possible DSs, as shown in Figure 3.
no matter which head words we choose for the internal nodes in the PS, the resulting DSs will not be the ones in Figure 4.