 reserve U for set,
         X, Y for Subset of U;
 reserve U for non empty set,
         A, B, C for non empty IntervalSet of U;

theorem Th16:
  A``1 c= A``2
  proof
      consider B being Subset of U such that A1: A = Inter (A``1,B) by Def5;
      consider C being Subset of U such that A2: A = Inter (C,A``2) by Def6;
      A``1 = C by Th6,A2,A1;
      hence thesis by Th5,A2;
  end;
