sentences:  Notice how the kludge codice_9 has been used twice because codice_6 was not explicitly defined.One of the problems with BNF which this example illustrates is that by allowing both single-quote and double-quote characters to be used for a codice_11, there is an added potential for human error in attempting to create a machine-readable syntax.One of the concepts migrated to later metasyntaxes was the idea that giving the user multiple choices made it harder to write parsers for grammars defined by the syntax, so computer languages in general have become more restrictive in how a "quoted-literal" is defined.