So the only condition for adding such a tree set is that the verb it depends on has already been added, since the tree of this verb provides the substitution node for the initial tree.
since the lexical material is always left of the foot node, one obtains that Note that in Figure 7, for a scrambled n i , the substitution node is filled with an empty node, while the n is adjoined higher at a node that is not yet available in the elementary structure of v i .