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