 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 Bsymmetry:
  between a,p,z implies between z,p,a
  proof
    assume
H1: between a,p,z;
    between p,z,z by Bqaa; then
    consider x such that
X1: between p,x,p & between z,x,a by H1, A7;
    thus thesis by X1, A6;
  end;
