theorem
  LSeg(W-min X, W-max X) c= LSeg(SW-corner X, NW-corner X)
proof
A1: (SW-corner X)`1 = W-bound X & (NW-corner X)`1 = W-bound X by EUCLID:52;
A2: (W-max X)`2 <= (NW-corner X)`2 by Th30;
  (W-max X)`1 = W-bound X & (SW-corner X)`2 <= (W-max X)`2 by Th30,EUCLID:52;
  then
A3: W-max X in LSeg(SW-corner X, NW-corner X) by A1,A2,GOBOARD7:7;
A4: (W-min X)`2 <= (NW-corner X)`2 by Th30;
  (W-min X)`1 = W-bound X & (SW-corner X)`2 <= (W-min X)`2 by Th30,EUCLID:52;
  then W-min X in LSeg(SW-corner X, NW-corner X) by A1,A4,GOBOARD7:7;
  hence thesis by A3,TOPREAL1:6;
end;
