theorem Th4:
  g is strict left-right
proof
  consider alpha such that
A1: g in ConwayDay(alpha) by Def3;
  ex w being strict left-right st w = g &
    for x st x in (the LeftOptions of w) \/ (the RightOptions of w)
      ex beta st beta in alpha & x in ConwayDay(beta) by A1,Th1;
  hence thesis;
end;
