theorem Th57:
  H is conjunctive implies (F is_immediate_constituent_of H iff F
  = the_left_argument_of H or F = the_right_argument_of H)
proof
  assume H is conjunctive;
  then H = (the_left_argument_of H) '&' the_right_argument_of H by Th40;
  hence thesis by Th53;
end;
