reserve S for Subset of TOP-REAL 2,
  C,C1,C2 for non empty compact Subset of TOP-REAL 2,
  p,q for Point of TOP-REAL 2;
reserve i,j,k for Nat,
  t,r1,r2,s1,s2 for Real;
reserve D1 for non vertical non empty compact Subset of TOP-REAL 2,
  D2 for non horizontal non empty compact Subset of TOP-REAL 2,
  D for non vertical non horizontal non empty compact Subset of TOP-REAL 2;

theorem Th60:
  N-bound L~SpStSeq C = N-bound C
proof
  set S1 = LSeg(NW-corner C,NE-corner C), S2 = LSeg(NE-corner C,SE-corner C),
  S3 = LSeg(SE-corner C,SW-corner C), S4 = LSeg(SW-corner C,NW-corner C);
A1: (NW-corner C)`2 = N-bound C by EUCLID:52;
A2: S-bound C <= N-bound C by Th22;
A3: S3 \/ S4 is compact by COMPTS_1:10;
A4: (SW-corner C)`2 = S-bound C by EUCLID:52;
  then
A5: N-bound S4 = N-bound C by A1,Th22,Th56;
A6: (SE-corner C)`2 = S-bound C by EUCLID:52;
A7: (NE-corner C)`2 = N-bound C by EUCLID:52;
  then
A8: N-bound S2 = N-bound C by A6,Th22,Th56;
A9: N-bound(S1 \/ S2) = max(N-bound S1,N-bound S2) by Th49
    .= max(N-bound C,N-bound C) by A1,A7,A8,Th56
    .= N-bound C;
A10: L~SpStSeq C = (S1 \/ S2) \/ (S3 \/ S4) by Th41;
A11: S1 \/ S2 is compact by COMPTS_1:10;
  N-bound(S3 \/ S4) = max(N-bound S3,N-bound S4) by Th49
    .= max(S-bound C,N-bound C) by A6,A4,A5,Th56
    .= N-bound C by A2,XXREAL_0:def 10;
  hence N-bound L~SpStSeq C = max(N-bound C,N-bound C) by A10,A11,A9,A3,Th49
    .= N-bound C;
end;
