Constraints on the form factors for K --> pi l nu and implications for   V_us
  Rigorous bounds are established for the expansion coefficients governing the shape of semileptonic K-->pi form factors. The constraints enforced by experimental data from tau-->K pi nu eliminate uncertainties associated with model parameterizations in the determination of |V_us|. The results support the validity of a powerful expansion that can be applied to other semileptonic transitions. 
Richard J. Hill (Fermilab)