When an invention is made, the inventor has three possible courses of action open to him: he can give the invention to the world by publishing it, keep the idea secret, or patent it. A granted patent is the result of a bargain made between an inventor and the state, by which the inventor gets a limited period of monopoly (垄断) and publishes full details of his invention to the public after that period terminates (终止). Only in most exceptional circumstances is the life-span of a patent extended to alter this normal process of events. The longest extension ever granted was to Georges Valensi: his 1939 patent for color TV receiver circuitry was extended until 1971 because for most of the patent's normal life there was no color TV to receive and thus no hope of reward for the invention. Because a patent remains permanently public after it has terminated, the shelves of the library attached to the patent office contain details of literally millions of ideas that are free for anyone to use and, if older than half a century, sometimes even re-patent. Indeed, patent experts often advise anyone wishing to avoid the high cost of conducting a search through live patents that the one sure way of avoiding violation of any other inventor's right is to plagiarize a dead patent. Likewise, because publication of an idea in any other form. permanently invalidates (使无效) further patents on that idea, it is traditionally safe to take ideas from other areas of print. Much modern technological advance is based on these presumptions of legal security. Anyone closely involved in patents and inventions soon learns that most 'new' ideas are, in fact, as old as the hills. It is their reduction to commercial practice, either through necessity of dedication, or through the availability of new technology, that makes news and money. The basic patent for the theory magnetic recording dates back to 1886. Many of the original ideas behind television originate from the late 19th and early 20th century. Even the Volkswagen rear engine car was anticipated by a 1904 patent of a cart with the horse at the rear. The passage is mainly about ______ .