In the traditional statement of T, sources(r) are the variables x i , standing for the i th child of the root at path (i), and the right hand sides of rules refer to them by name: (q i , x i ).
we can simulate lookahead using states, as in these productions:  By omitting rules like qpro NP → ..., we ensure that the entire production sequence will dead-end unless the first child of the input tree is in fact PRO.