theorem Th24:
  F is Until 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 Until;
  then F = (the_left_argument_of F) 'U' (the_right_argument_of F) by Th8;
  hence thesis by Th17;
end;
