1. F. Calogero andA. Degasperis:Nuovo Cimento,32 B, 201 (1976) (hereafter referred to as I).
2. Here, and always in the following, a subscripted variable indicates partial differentiation.
3. V. E. Zakharov andA. B. Shabat:Funk. Anal. Appl.,8, 43 (1974).
4. See the review paper byV. E. Zakharov in the forthcoming Springer monograph edited byR. K. Bullough.
5. F. Calogero:Lett. Nuovo Cimento,14, 537 (1975).