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