Aminoguanidine and alkoxyguanidine compounds, including compounds of the formula: . A light incidence surface of the light guide plate receiving a light from a lamp is inclined to make an obtuse angle with a light-emitting surface through which the light is transferred to the display unit. A control variable is measured, the value of the control variable is compared to a comparison value, and in the case of deviations of the value of the control variable from the comparison value, at least one parameter of the transfer function is corrected. A difference between the two estimates is an indication that the sands may be anisotropic. The second gap is less than the first gap to facilitate restraining the vehicle to the trailer.