# File lib/em/deferrable.rb, line 43
43:     def callback &block
44:       return unless block
45:       if @deferred_status == :succeeded
46:         block.call(*@deferred_args)
47:       elsif @deferred_status != :failed
48:         @callbacks ||= []
49:         @callbacks.unshift block # << block
50:       end
51:     end