theorem Th23:
  F is disjunctive implies (H is_immediate_constituent_of F iff H
  = the_left_argument_of F or H = the_right_argument_of F)
proof
  assume F is disjunctive;
  then F = (the_left_argument_of F) 'or' (the_right_argument_of F) by Th7;
  hence thesis by Th16;
end;
