class ActiveSupport::Callbacks::Callback

def merge(chain, new_options)
  options = {
    :if     => @if.dup,
    :unless => @unless.dup
  }
  options[:if].concat     Array(new_options.fetch(:unless, []))
  options[:unless].concat Array(new_options.fetch(:if, []))
  self.class.build chain, @filter, @kind, options
end