 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 EquivSymmetric:
  a,b equiv c,d implies c,d equiv a,b
  proof
    assume
H1: a,b equiv c,d;
    a,b equiv a,b by EquivReflexive;
    hence thesis by H1, A2;
  end;
