theorem Th67:
  N-most L~SpStSeq C = LSeg(NW-corner C,NE-corner C)
proof
  set X = L~SpStSeq C;
  set S1 = LSeg(NW-corner C,NE-corner C), S2 = LSeg(NE-corner C,SE-corner C);
A1: S1 c= S1 \/ S2 by XBOOLE_1:7;
  X = (S1 \/ S2) \/ (LSeg(SE-corner C,SW-corner C) \/ LSeg(SW-corner C,
  NW-corner C)) by Th41;
  then
A2: S1 \/ S2 c= X by XBOOLE_1:7;
  LSeg(NW-corner X, NE-corner X) = LSeg(NW-corner X, NE-corner C) by Th63
    .= LSeg(NW-corner C,NE-corner C) by Th62;
  hence thesis by A1,A2,XBOOLE_1:1,28;
end;
