  Free differential algebras (FDA's) provide an algebraic setting for field
theories with antisymmetric tensors. The "presentation" of FDA's generalizes
the Cartan-Maurer equations of ordinary Lie algebras, by incorporating p-form
potentials. An extended Lie derivative along antisymmetric tensor fields can be
defined, and used to recover a Lie algebra dual to the FDA, that encodes all
the symmetries of the theory including those gauged by the p-forms.
  The general method is applied to the FDA of D=11 supergravity: the resulting
dual Lie superalgebra contains the M-theory supersymmetry anticommutators in
presence of 2-branes.
