 reserve S for satisfying_Tarski-model TarskiGeometryStruct;
 reserve a, b, c, d, e, f, o, p, q, r, s,
    v, w, u, x, y, z, a9, b9, c9, d9, x9, y9, z for POINT of S;

theorem A6:
  between a,b,a implies a = b
  proof
    S is satisfying_BetweennessIdentity;
    hence thesis;
  end;
