Two-level grammar
A two-level grammar is a formal grammar that is used to generate another formal grammar , such as one with an infinite rule set . This is how a Van Wijngaarden grammar was used to specify Algol 68 . A context free grammar that defines the rules for a second grammar can yield an effectively infinite set of rules for the derived grammar. This makes such two-level grammars more powerful than a single layer of context free grammar, because generative two-level grammars have actually been shown to be Turing complete.
Two-level grammar can also refer to a formal grammar for a two-level formal language, which is a formal language specified at two levels, for example, the levels of words and sentences.Example
A well-known non-context-free language is
A two-level grammar for this language is the metagrammar
together with grammar schema
