Using • to indicate left-most substitution, we can write: • t 1 is a root-subtree of t 1 , and t 1 is a root-subtree of t 2 , if ∃t 3 , such that t 3 • t 1 = t 2 ; • t 1 is a frontier-subtree of t 1 , and t 1 is a frontier- • t is the x-frontier-subtree of t, t = f s x is a set of nodes in t, such that if t is pruned at each i ∈ x it equals t .
given an initial setting of the parameters (all depth 1 elementary trees at their empirical frequency), the method calculates the expected frequency of all complete and incomplete trees.