def constraint(self, n=-1, fid=0):
        """Obtain the set of orthogonal equations that make the solution of
        the rank deficient normal equations possible.

        :param fid: the id of the sub-fitter (numerical)

        """
        c = self._getval("constr", fid)
        if n < 0 or n > self.deficiency(fid):
            return c
        else:
            raise RuntimeError("Not yet implemented")
Obtain the set of orthogonal equations that make the solution of
        the rank deficient normal equations possible.

        :param fid: the id of the sub-fitter (numerical)