Early formal specifications of natural language syntax were quite closely connected to the notion of abstract machines for computing them. More recently, this approach has been superseded by one in which languages are specified in terms of systems of constraints on the structure of their sentences. This has made complexity results difficult to obtain. This book introduces a way of obtaining such results. It presents a natural and quite general means of expressing constraints on the structure of trees and shows that the languages that can be specified by systems of such constraints are exactly...