def output_port(name)
      raise NoSuchPortError.new("output port \"#{name}\" not found") unless output_ports.has_key?(name)
      output_ports[name]
    end
def output_port(name)
      raise NoSuchPortError.new("output port \"#{name}\" not found") unless output_ports.has_key?(name)
      output_ports[name]
    end
Find the output port identified by the given *name*