One key point illustrated by Figure 11 is that whereas multiple adjunction operates on a single node (here η o ), the unification constraints of FB-TAG require that the bottom feature structure of the foot of an auxiliary tree which appears higher in the derived tree (here, β 2 ) unifies with the bottom feature structure of the root of the auxiliary tree appearing immediately below it in the derived tree (here β 1 )-not with that of the root of the node to which it adjoins (here η o ).
while a multiple adjunction on η o operates on η o only, a correct implementation of FB-TAG unification constraints requires keeping track of the feature structures associated with the auxiliary trees successively adjoining to η o .