More generally, each elementary tree in the grammar is associated with the set of classes used to produce that tree and importantly, this set of classes (we will call this the tree identifier) provides a distinguishing description (a unique identifier) for that tree: a tree is defined by a specific combination of classes and conversely, a specific combination of classes yields a unique tree.
4 the set of classes associated by the compilation process with a given elementary tree can be used to uniquely identify that tree.