We consider the possibility that one extra $Zequiv Z$ exists with arbitrary mass and fermion couplings that do not violate (charged) lepton universality. We show that, in such a situation, a functional relationship is generated between the underline{deviations} from the SM values of three leptonic observables of two-fermion production at future $e^+e^-$ colliders that is completely independent of the values of the $Z$ mass and couplings. This selects a certain region in the 3-d space of the deviations that is underline{characteristic} of the model ($Z$ reservation). As a specific and relevant example, we show the picture that would emerge at LEP2 under realistic experimental conditions.
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