Double Bar. The double bar symbol is used to denote certain kinds of norms in mathematics (e.g., or). It is also used to denote parallel lines, as in, and in an older notation for the covariant derivative. In this work, the notation or is reserved for matrix and function norms, respectively.
The F-4 Phantom II remained in use by the U.S. in the reconnaissance and Wild Weasel (Suppression of Enemy Air Defenses) roles in the 1991 Gulf War, finally leaving service in 1996. It was also the only aircraft used by both U.S. flight demonstration teams: the USAF Thunderbirds (F-4E) and the US Navy Blue Angels (F-4J).
Previous strikes in Libya used drones and F-15s from a U.S. Air Force base in England, and two Navy guided-missile destroyers were off the coast of Libya armed with Tomahawk cruise missiles. B-2 Spirit stealth bombers had last seen combat in March 2011, helping to enforce a no-fly zone over Libya, Pentagon officials said. The B-2 Spirit is the world's most technologically-advanced strategic bomber and brings massive firepower to bear, in a short time, anywhere on the globe, Air Force spokesman Capt. Mark Graff told reporters.