  We present a review of the homological algebra tools involved in the standard
de Rham theory and their subsequent generalizations relevant for the
understanding of free massless higher spin gauge structure. M-theory arguments
suggest the existence of an extension of (Abelian) S-duality symmetry for
non-Abelian gauge theories, like the four dimensional Yang-Mills or Einstein
theories. Some no-go theorems prove that this extension, if it exists, should
fall outside the scope of local perturbative field theory.
