def each_by_param(param_name)
      REXML::XPath.each(self, lookup_param(param_name).path) { |e| yield e }
    end
def each_by_param(param_name)
      REXML::XPath.each(self, lookup_param(param_name).path) { |e| yield e }
    end
Yields up each XML element for the given Param object.