def check
      @definition.namespaces.each do |(namespace, ns)|
        ns.items.each do |key, _|
          lookup(namespace, key)
        end
      end
    end
def check
      @definition.namespaces.each do |(namespace, ns)|
        ns.items.each do |key, _|
          lookup(namespace, key)
        end
      end
    end
check items have a value. Will raise Undefined error if a required item
 has no value.