theorem Th22:
  F is conjunctive 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 conjunctive;
  then F = (the_left_argument_of F) '&' (the_right_argument_of F) by Th6;
  hence thesis by Th15;
end;
