function df_da = drelerr_snn(a, t)%DRELERR_SNN 1st derivative of RELERR_SNN.%%  Syntax%%    df_dy = drelerr_snn(y0, t0)%%     y0    - output%     t0    - target%     df_dy - derivative of RELERR_SNN(y,t) with respect to y%             in (y0, t0)%% See also%%   RELERR_SNN%df_da = (a./t - 1)./t;