theorem
  H is conjunctive implies the_left_argument_of H
  is_proper_subformula_of H & the_right_argument_of H is_proper_subformula_of H
proof
  assume H is conjunctive;
  then the_left_argument_of H is_immediate_constituent_of H &
  the_right_argument_of H is_immediate_constituent_of H by Th57;
  hence thesis by Th61;
end;
