1. The selection of the following references is to a large extent arbitrary
2. S. Okubo: Progr. Theor. Phys. 11, 80 (1954)
3. R. Arnowitt and S. Deser: Phys. Rev. 100, 349 (1955)
4. GV. Efimov : Soviet. Phys. JETP 44, 2107 (1963) TH. 1087 - CERN (1969)
5. E. S. Fradkin: Nucl. Phys. 49, 624 (1963)