 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;
 reserve K,L,M for Element of RoughSets X;

theorem Th69:
  for A being RoughSet of X holds ERS X _\/_ A = A;
