“…In that sense (12) is dual to (13), (14) dual to (16), (17) dual to (20), (18) dual to (19) while (15) is a self-dual equation. Equation (12) appears to be new, at least to the authors' knowledge, while the case of (15) will be examined in the discussion. On the other hand (14), (17) and (18) were identified [2], in their autonomous forms, as being linearisable.…”