4.1 Descriptions of Trees
Since trees are a basic food group in the computational linguist's diet, it is often very convenient to be able to describe classes of trees with certain general structural properties. As mentioned earlier, a tree is typically specified extensionally by listing its edges, i.e. by providing a relation of immediate dominance between vertices. This is often too specific, hence the idea of also permitting the more relaxed relation of (general) dominance, which is simply the transitive closure of immediate dominance.
This makes it possible to express that some vertex 
should dominate another vertex 
, which we write 
, without having to commit to any specific detail. This merely requires that must occur below , and allows arbitrary material to be inserted on the path between them. In other words, math/v_1\LE v_2/ is satisfied in every model (tree) in which denotes a proper ancestor of .