Structural rule A famous structural rule is known as cut. Considerable effort is spent by proof theorists in showing that cut rules are superfluous in various logics. More precisely, what is shown is that cut is only (in a sense) a tool for abbreviating proofs, and does not add to the theorems that can be proved. The successful 'removal' of cut rules, known as "cut elimination", is directly related to the philosophy of "computation as normalization" (see Curry–Howard correspondence); it often gives a good indication of the complexity of deciding a given logic. Structural rule In proof theory, a structural rule is
Two-third rule than freight trains. On such tracks, the two-third rule applies to freight trains. The two-third rule means that the speed limit for a freight train is generally about two-thirds of the speed limit for an EMU. For example, the Dartford Loop Line has a speed limit of 60 mph for EMUs between Lee and Crayford. A freight train on this track is therefore limited to two-thirds of 60 mph, i.e. 40 mph. Two-third rule Two-third rule is British railway safety rule. For safety reasons, speed limits are imposed on trains running on a railway line. Many factors can affect the