 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;
 reserve X for Tolerance_Space,
         A, B, C for RoughSet of X;

theorem Th67:
  A _\/_ (A _/\_ B) = A by XBOOLE_1:22;
