There is an extended grammar notation in common use. In this notation, square and curly braces in production bodies are metasymbols (like $\rightarrow$ or $\mid$) with the following meanings:
- Square braces around a grammar symbol or symbols denotes that these constructs are optional. Thus, production $A\rightarrow X [Y] Z$ has the same effect as the two productions $A\rightarrow X Y Z$ and $A\rightarrow X Z$.
- Curly braces around a grammar symbol or symbols says that these symbols may be repeated any number of times, including zero times. Thus,$A\rightarrow X \{Y Z\}$ has the same effect as the infinite sequence of productions $A\rightarrow X,A\rightarrow X Y Z,A\rightarrow X Y Z Y Z$,and so on.
Show that these two extensions do not add power to grammars; that is, any language that can be generated by a grammar with these extensions can be generated by a grammar without the extensions.