1. A method of comparing the closeness of a target tree to other trees located in a database of trees, said method comprising the steps of: (a) calculating a constraint in respect of each tree in the database based on an estimated number of edit operations and a characteristic of the target tree; (b) calculating a constrained tree edit distance between the target tree and each tree in the database using the constraint obtained in step (a); and (c) comparing the calculated constrained tree edit distances.