theorem Th14:
  LSeg(NW-corner C,NE-corner C) c= L~SpStSeq C
proof
A1: LSeg(NW-corner C,NE-corner C) \/ LSeg(NE-corner C,SE-corner C) c= (LSeg(
NW-corner C,NE-corner C) \/ LSeg(NE-corner C,SE-corner C)) \/ (LSeg(SE-corner C
  ,SW-corner C) \/ LSeg(SW-corner C,NW-corner C)) by XBOOLE_1:7;
  LSeg(NW-corner C,NE-corner C) c= LSeg(NW-corner C,NE-corner C) \/ LSeg(
  NE-corner C,SE-corner C) by XBOOLE_1:7;
  then
  LSeg(NW-corner C,NE-corner C) c= (LSeg(NW-corner C,NE-corner C) \/ LSeg(
NE-corner C,SE-corner C)) \/ (LSeg(SE-corner C,SW-corner C) \/ LSeg(SW-corner C
  ,NW-corner C)) by A1;
  hence thesis by SPRECT_1:41;
end;
