1. For a recent review see M.J. Duff, Lecture at the Spring school on supergravity in “Supergravity 81” ed. S. Ferrara and J.G. Taylor (Camb. Univ. Press)
2. G. t'Hooft, in Recent developments in gravitation, 1978 Cargèse lectures, p. 333, ed. M. Lévy and S. Deser (Plenum Press, New York)
3. Nonlinear Realizations of Chiral Symmetry