  The concept of exchange in strongly-correlated fermions is reviewed with
emphasis on the generalization of the Heisenberg pair exchange to higher order
n-particle permutations. The "frustration" resulting from competing
ferromagnetic three-spin exchange and antiferromagnetic two- and four-spin
exchanges is illustrated on a two-dimensional model system: solid 3He films.
Recent experimental results proving the presence of four-spin exchange
interactions in the CuO2 plaquettes of high Tc cuprates are reported.
