where the variables are differential forms W A = W μ μ 1 . . . μ μ q A ( x ) d x μ μ 1 ∧ ∧ . . . ∧ ∧ d x μ μ q of various degrees, enumerated by an abstract index A ; d is the exterior derivative d = d x μ μ ∂ ∂ μ μ . The structure function F A ( W ) is assumed to be expandable in exterior product Taylor series as