theorem
  H is conjunctive implies Subformulae H = Subformulae
  the_left_argument_of H \/ Subformulae the_right_argument_of H \/ { H }
proof
  assume H is conjunctive;
  then H = (the_left_argument_of H) '&' the_right_argument_of H by Th3;
  hence thesis by Th89;
end;
