Prepare a metamodel of Backus-Naur (BNF) representations of computer languages. A compiler-compiler (such as the UNIX program YACC) could use the model. The compiler- compiler accepts these representations in graphical form as input and produces a compiler for the represented language. Figure E4.5 shows an example of a Backus-Naur form that the compiler-compiler will ac- cept. Rectangles denote nonterminals, and circles or rectangles with rounded corners denote ter- minals. Single characters are in circles, and sequences of several characters are in rounded rect- angles. Arrows indicate the direction of flow through the diagram. Where several directed paths diverge, it is permissible to take any one of them. The name of the nonterminal being described appears at the beginning of its representation.

name NAME string string " character Figure E4.5 Portion of a BNF diagram name NAME string string " character Figure E4.5 Portion of a BNF diagram