An extended BNF for specifying the syntax of declarations

The syntax of ALGOL is specified in its defining report by a grammar whose rules are given in Backus-Naur Form (BNF). Because of the need for precision in the specification of complex systems, BNF has been used to record the syntax of many other programming languages. The rules of BNF are equivalent to context-free production rules. However, due to its declarations, ALGOL is context-dependent, and its syntax cannot be fully specified by a grammar limited to context-free rules. Other grammars for ALGOL are more compact than the BNF of the defining report, but they are generatively no more powerful. Because of this context-dependency of declarations, even an assembly language cannot be fully specified in BNF.