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 Th73:
  proj1.:LSeg(SE-corner C,SW-corner C) = [.W-bound C,E-bound C.]
proof
A1: (SW-corner C)`1 = W-bound C by EUCLID:52;
  (SE-corner C)`1 = E-bound C by EUCLID:52;
  hence thesis by A1,Th21,Th52;
end;
