def set_mark(key, value, dup=true)
      self.each {|elm| elm.set_mark(key, (dup and not(Symbol===value)) ? value.dup : value)}
    end
def set_mark(key, value, dup=true)
      self.each {|elm| elm.set_mark(key, (dup and not(Symbol===value)) ? value.dup : value)}
    end
Fired to each element of the group. Values are duplicated by default.
 Put dup to false to avoid this behavior.