Suppose the grammar in this example is modified as follows: Rules for lexical phrases are applied to X 0 These modifications exclude parses in which hierarchical translation rules generate other hierarchical rules, except at the 0 th level which generate lexical phrases.
the left most tree of Figure 10 cannot be generated and t 5 t 1 t 2 t 7 is the only allowable translation of s 1 s 2 s 3 s 4 .