        the local regression model is fitted as
                        Y_i = b_i + m(x_i) + epsilon_i,
           with Locfit estimating the m(x) term. For regression models,
           this effectively subtracts b_i from Y_i. The advantage of the
           'base' formulation is that it extends to likelihood
           regression models. 
    'scale' A scale to apply to each variable. This is especially
           important for multivariate fitting, where variables may be
           measured in no