Tree nodes form a tree, which means that two nodes at the same level in an FTF must have a common parent node. FTFs employ two broad classes of structural relationship. Unary relationships are structural properties of a single tree node or text unit element. These include the statement that a node is, or is not, the root , a leaf , or the first or last child in a sequence of child nodes. For text unit elements, they specify whether they are the first or last word in the sentence.
Binary relationships relate the position of one tree node or text unit element to another.
The parent of a node in an FTF is either immediately or eventually connected. The next child in a sequence of children may be just after another, just before or just after , following after , or before or after , or the relationship may be unknown. Text unit elements take a parallel set of relations. Finally, one may specify that two tree nodes must be located on different branches. We have found that this definition of FTFs is intuitive, yet sufficiently expressive for highly specialised queries.