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;
