which are usually classified according to intent: Watermarks are used to show that items are authentic and not forged. Steganography is used to hide a secret message in an apparently innocuous message, in order to escape detection. A canary trap hides information in a document that uniquely identifies it, so that copies of it can be traced. Screener versions of DVDs are often marked in some way so as to allow the tracking of unauthorised releases to their source. As with the Star Trek incident, major films or television productions frequently give out scripts to the cast and crew in which one or two lines are different in each individual version. Thus if the entire script is copied and leaked to the public, the producers can track down the specific person who leaked the script. In practice this does not prevent generalized information about the script from being leaked, but it does discourage leaking verbatim copies of the script itself. Trap streets on maps, or intentionally fictitious streets, are sometimes included to track copyright violations by those who might republish copyrighted maps illegally. Spurious words are sometimes included in dictionaries so as to detect other publishers copying from them. The Oxford English Dictionary contains an appendix of such words with which edition of which dictionary first used them and which first duplicated them. Zero-width spaces are Unicode characters that are not visually rendered. An arbitrary number of these characters can be inserted between the letters of a word. Though they are not visible, they will typically persist even as that word is copied and pasted and transmitted multiple times. This can be used to create persistent, invisible fingerprints in digital text. Mailing lists for purchase are typically seeded with a small number of postal or e-mail addresses that route back to the seller of the list. This is used to detect re-use of the list (typically sold on a per-mailing basis) or resale of the list (usually prohibited). In popular culture The canary trap was used in several of Tom Clancy's novels. Chronologically it first appears in Without Remorse, when a CIA official alters a report given to a senator, revealing an internal leak who was giving information to the KGB. Different versions of the report were given to other suspected leakers. Barium meals are also administered in Robert Littel's book The Company, and later in the TV miniseries with