 Another line along which the notion has been broadened (or along which some have sought to broaden it) involves recent work on  substructural logic.  The proposal here is that we may consider doing without some of the standard rules governing the way that premises (or conclusions) of an argument may be combined. Structural rules deal with the shape or structure of an argument in the sense of the way that the premises and conclusions are collected together, and not the way that those statements are constructed. The structural rule of weakening for example, states that if an argument from some collection of premises \(X\) to a conclusion \(C\) is valid, then the argument from \(X\) together with another premise \(A\) to the conclusion \(C\) is also valid. This rule has seemed problematic to some (chiefly on the grounds that the extra premise \(A\) need not be used in the derivation of the conclusion \(C\) and hence, that \(C\) does not follow from the premises \(X,A\) in the appropriate sense).  Relevant logics  are designed to respect this thought, and do without the structural rule of weakening. (For the proof-theoretic picture, see Negri and von Plato (2001).) 