class Rouge::Lexer

def demo_file(arg=:absent)

this lexer (can be overriden by {demo}).
Specify or get the path name containing a small demo for 
def demo_file(arg=:absent)
  return @demo_file = Pathname.new(arg) unless arg == :absent
  @demo_file = Pathname.new(File.join(__dir__, 'demos', tag))
end