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