Every context-free grammar G has an equivalent grammar in Chomsky Normal Form. Greibach Normal Form. Removal of left-recursion from a production rule. Every context-free grammar G has an equivalent grammar in Greibach Normal Form. Extended PDA model: allow pushing strings into the stack. Note that since the transition relation of a PDA is finite, the extended PDA model allows pushing of bounded-length strings only (i.e. there is a constant c such that every string pushed by the extended PDA is of length < c). This implies an extended PDA can be simulated by a regular PDA in the obvious way. From Greibach Normal Form to an extended PDA. A language is context-free iff it is accepted by a PDA.