merge_conditional_options

# File activesupport/lib/active_support/callbacks.rb, line 324
      def merge_conditional_options(chain, if_option,, unless_option))
        options = {
          :if     => @if.dup,
          :unless => @unless.dup
        }

        options[:if].concat     Array(unless_option)
        options[:unless].concat Array(if_option)

        self.class.build chain, @filter, @kind, options
      end