I've got VECM model with [3x3] matrix Pi=alpha*beta' that looks as follows: [-1, a1*b, a2*d; 0, b-1, 0; 0, 0, d-1]. a1, b, a2 and d are parameters. I was thinking about imposing restrictions for elements that equals 0. As I'm aware it's not possible to put such restrictions on matrix Pi (or am I wrong?). It's better to find sensible 'alpha' and 'beta' in this case (but I don't have any ideas yet )?