If we think of the parser as dynamically converting G into a CFG G , then this CFG is likewise able to compositionally reconstruct TSG, TIG, or RF-TAG derivations-we say that G simulates G. Note that the parser specifies how to convert G into G , but G is not itself a parser.
these three formalisms have a special relationship to CFG that is independent of any particular parsing algorithm: for any TSG, TIG, or RF-TAG G, there is a CFG that simulates G. We make this notion more precise below.