The diagram (5) is an informal representation of what we may call a 'Transformation-marker'. It represents the transformational structure of the utterance (5) very much in the way a Phrase-marker represents the phrase structure of a terminal string. In fact, a Transformation-marker may be formally represented as a set of strings in an alphabet consisting of base Phrase-markers and transformations as its elements [].
- Chomsky (1969), Pag. 131