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__).dirname.join('demos', tag)
end