# File lib/test/unit/util/observable.rb, line 31
        def add_listener(channel_name, listener_key=NOTHING, &listener) # :yields: value
          unless(block_given?)
            raise ArgumentError.new("No callback was passed as a listener")
          end
      
          key = listener_key
          if (listener_key == NOTHING)
            listener_key = listener
            key = ProcWrapper.new(listener)
          end
      
          channels[channel_name] ||= {}
          channels[channel_name][key] = listener
          return listener_key
        end