theorem Th99:
  variables_in Non a = variables_in a proof per cases;
  suppose a is non positive;
    then consider a9 being expression of C, an_Adj C such that
A1: a = (non_op C)term a9 and
A2: Non a = a9 by Th60;
    [non_op C, the carrier of C]-tree<*a9*> = a by A1,Th43;
    hence thesis by A2,Th93;
  end;
  suppose a is positive;
    then Non a = (non_op C)term a by Th59
      .= [non_op, the carrier of C]-tree <*a*> by Th43;
    hence thesis by Th93;
  end;
end;
