 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 Th64:
  A _\/_ B _\/_ C = A _\/_ (B _\/_ C) by XBOOLE_1:4;
