code: def constraint(self, n=-1, fid=0):
          
          c = self._getval("constr", fid)
          if n < 0 or n > self.deficiency(fid):
              return c
          else:
              raise RuntimeError("Not yet implemented")
docs: 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)