after a short discussion on the mailing list, saved here so that it isn't forgotten (again, this is not urgent and I'm glad nonlinear restrictions have been enabled at all):
<Allin:> If you do that, please include a detailed design suggestion.
<Sven:> Well correct me if I'm wrong, but constructing the auxiliary restriction
function which returns a zero matrix under the null hypothesis is purely
algorithmic. So if you have a restriction as in the above example,
b[LRY]*b[IBO] - 2*b[IDE]*b[LRY] = 0
it directly translates into:
matrix h0 = b*b-2*b*b
(well the ordering of the coeffs needs to be taken into account of
course) and the rest is already implemented in gretl. And I think it's
completely ok to require that nonlinear restrictions should be given
with a zero on the right-hand side.