code: public String nextLine(Map<String,Object> extraClasses) {
          return mainClass.render(null, preprocessExtraClasses(extraClasses));
      }
docs: Returns a random line of text, driven by the underlying spew file and the given
  classes. For example, to drive a spew file but add some parameters to it,
  call this method with the class names as the map keys, and the Strings that you'd
  like substituted as the values.
  
  @param extraClasses the extra classes to use
  @return a random line of text