If X ∈ W (i.e., if X is a terminal) then T X = {X}, i.e., T X consists of a single tree, which in turn only consists of a single node labelled X.
if X ∈ N (i.e., if X is a nonterminal) then where R A = {A → β : A → β ∈ R} for each A ∈ N , and TREE X (T B 1 , .