pplied attribute as the key to be looked
            up in the internal associative container. If the key exists the generator
            emits the associated value and succeeds (unless the underlying output
            stream reports an error). If the value type stored in the symbol generator
            is <code class="computeroutput"><span class="identifier">unused_type</span></code> it will
            emit the key instead. If the key does not exist the generator fails while
            not emitting an