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Nonterminals are well known from parsers where they are used as the main means of constructing more complex parsers out of simpler ones. The nonterminals in the parser world are very similar to functions in an imperative programming language. They can be used to encapsulate parser expressions for a particular input sequence. After being defined, the nonterminals can be used as 'normal' parsers in more complex expressions whenever the encapsulated input needs to be recognized. Parser nonterminals in Spirit.Qi may accept parameters (inherited attributes) and usually return a value (the synthesized attribute).
          Both, the types of the inherited and the synthesized attributes have to
          be explicitly specified while defining the particular grammar
          or the rule (the Spirit
          Repository additionally
          has subrules which conform
          to a similar interface). As an example, the following code declares a
          Spirit.Qi rule
          exposing an int as its synthesized
          attribute, while expecting a single double
          as its inherited attribute (see the section about the Spirit.Qi
          Rule for more information):
        
qi::rule<Iterator, int(double)> r;
          In the world of generators, nonterminals are just as useful as in the parser
          world. Generator nonterminals encapsulate a format description for a particular
          data type, and, whenever we need to emit output for this data type, the
          corresponding nonterminal is invoked in a similar way as the predefined
          Spirit.Karma generator primitives. The Spirit.Karma
          nonterminals are very similar to the Spirit.Qi nonterminals.
          Generator nonterminals may accept parameters as well,
          and we call those inherited attributes too. The main difference is that
          they do not expose a synthesized attribute (as parsers do), but they require
          a special consumed attribute. Usually the consumed
          attribute is the value the generator creates its output from. Even if the
          consumed attribute is not 'returned' from the generator we chose to use
          the same function style declaration syntax as used in Spirit.Qi.
          The example below declares a Spirit.Karma rule consuming a double
          while not expecting any additional inherited attributes.
        
karma::rule<OutputIterator, double()> r;
The inherited attributes of nonterminal parsers and generators are normally passed to the component during its invocation. These are the parameters the parser or generator may accept and they can be used to parameterize the component depending on the context they are invoked from.