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-barrier rule Two-barrier rule The two barrier rule is a policy of safety for work on piping carrying high pressure and/or high temperature fluids. It is commonly used by oil companies. The rule simply states that for any work that must be done on a section of piping, which involves breaking containment, there must be at least two barriers separating the fluids from the broken area. The purpose behind specifying two barriers is to ensure redundancy. The risk of a total loss of containment is reduced if there are two independent isolations as it is highly improbable they would both fail at