reserve a,b,r for non unit non zero Real;

theorem
  op2(op1(op2(op1(r)))) = op1(op2(r)) &
  op1(op2(op1(op2(r)))) = op2(op1(r))
  proof
    op2(op1(op2(op1(r)))) = 1 - (r / (r - 1)) &
    op1(op2(op1(op2(r)))) = 1 / (r / (r - 1)) by Th01;
    hence thesis;
  end;
