al representation
          of the parsed input (i.e. without 'producing' any data from the parsed
          input). Using a parser in this mode checks the provided input against the
          given parser expression allowing to verify whether the input is parsable.
          For generators this mode doesn't make any sense. What is output generation
          without generating any output? So we always will have to supply the data
          the output should be generated from. In our example we supply a lis