theorem Th17:
  [li,ri] is Gap of Gi & [li,ri9] is Gap of Gi implies ri = ri9
proof
A1: ri <= ri9 & ri9 <= ri implies ri = ri9 by XXREAL_0:1;
  assume that
A2: [li,ri] is Gap of Gi and
A3: [li,ri9] is Gap of Gi;
A4: ri in Gi by A2,Th13;
A5: ri9 in Gi by A3,Th13;
  per cases by A2,Th13;
  suppose
A6: li < ri & for xi st xi in Gi holds not (li < xi & xi < ri);
    ri9 <= li or li < ri9 & ri9 < ri or ri <= ri9;
    hence thesis by A1,A3,A4,A5,A6,Th13;
  end;
  suppose
A7: ri < li & for xi st xi in Gi holds not (li < xi or xi < ri);
    ri9 < ri or ri <= ri9 & ri9 <= li or li < ri9;
    hence thesis by A1,A3,A4,A5,A7,Th13;
  end;
end;
